There are thirty coins to be found in Gelato Beach. Many are there in every episode, the biggest exception being those in the sand bird's domain since you can only enter that in episode 4.
Sea floor (x2)
There are two blue coins sitting on the sea floor. First go to the small island with three palm trees that's behind you when you start. Climb the middle and, while facing out to sea, look down and to the left so spot a coin. Now go to the large, teardrop shaped peninsula and start swimming out to sea. When the water starts to get deep, dive and look around to find the other blue coin. It's difficult to be specific because there are no nearby landmarks, but the water is very clear and there is little in the way of clutter, so you should be able to see the coins easily even if you're not very close. Do not look for these coins in episodes 3 or 5 as they won't be there. Available episodes 1, 2, 4, 6, 7, 8.
Swimming with fish (x2)
Two more coins in the water are swimming along with schools of fish. If you've done episode 6 then you've seen this kind of thing with red coins. Since the fish swim around with a fairly broad range, it's impossible to give a specific location. So keep swimming around in the sea between the island with palm trees and the coral reef until you spot a school of blue fish for the first coin and brown fish for the second coin. All the fish are gone in episodes 3 and 5 so the coins don't appear either. Available episodes 1, 2, 4, 6, 7, 8.
Go to the island with the palm trees and climb the one on the right (as you're facing out to sea). This is the tallest one and has a swing hanging from it. Find a blue coin nestled in the crown. Available all episodes.
Swing the swing
From the swing mentioned in the treetop coin, notice an arc of gold coins with a blue coin at the top. Stand on the swing and use the squirt nozzle (with a nod to Newton's third law of motion) to start swing moving. Squirt in the opposite direction that you're going and just coast on the swing back and you should be swinging high enough to reach the coin in two or three squirts. Available all episodes.
From the start go straight towards the beach and keep going until you find a cabana, the sign next to it says it's known as the Sand Cabana. Hop on one of the seats and hop again to grab the ceiling. There is a blue coin in the center surrounded by four gold coins. Available all episodes.
From the start go left and keep going until you reach a boardwalk sticking out to sea near the end of the beach. At the end of the boardwalk is a cabana, the sign next to it says it's known as the Surf Cabana. In the back is a deck with some tables and umbrellas for an outdoor cafe. Drop into the water and swim under the deck, then swim around to collect the coins hiding there. There are seven gold coins and one blue coin. Available all episodes.
Follow the walk-through for episode 4 until you get to the small stadium. Depending on the episode there may or may not be a dune bud to help you up the cliff, if not then use wall jumps in the crack in the wall. Instead of sliding down the hill, follow the path that leads across it, use hover to get over the gaps since the hill is slippery. The path ends at a crevice between the side of the hill and a tiny plateau with a pole on top. Drop into the crevice to find a blue coin with gold coins on either side. The coins are above the ground a way so you may have to do a wall jump to reach them. Once you know where to look it may be easier to use the hover nozzle to push yourself up the slope to the crevice. Available all episodes.
Reflector coin stack
First, equip the rocket nozzle, then go to the beach near the Sand Cabana and climb one of the trees with light green trunks, the other trees are too spiky to climb. Follow the treetops to a wooden platform to the left of the main tower. Fire the rocket to collect gold coins and a blue coin at the top. You may have to hop just before the rocket goes off to get the extra few feet of altitude needed. It may not seem like it but it's possible to reach this coin without the rocket. To do it this way, jump on the trampoline next to the tower (see episode 4 walk-through) but now hover toward the tower and land on the ledges near the top. You should now be able to jump and hover from there to get the blue coin. Available all episodes.
High wire pole
There are bouncy wires in this level like the ones in Bianco Hills, but here the poles that hold them up don't have the handy little platforms on top. So to navigate them it helps to become skilled at bouncing from wire to wire. Another technique is to bounce to the top of a pole and then just drop to the next wire. There are also a number of gold coins floating above the wires that you can reach once you learn to do this. To get the blue coin, first equip the rocket attachment, then go to the wooden platform next to the reflector and continue toward the cliff to a small ledge. Fire to get to the lower wire attached to this pole. Then start charging the rocket and bounce just before it goes off to be launched to the upper wire. Bounce over the pole to the wire on the other side, then follow it to the end where you will see a blue coin at the top of the second support pole. Note that the only place you use the rocket here to get to the upper wire, and there are ways of doing this without the rocket. One alternative is to go to the end of the path mentioned in the hillside crevice coin, then back-flip and hover to the wire above it, and finally, bouncing three times for altitude, hover over to the upper wire. Available all episodes.
High wire coin stack
The other high wire coin is just a warm up compared to this one. Get to the high wire as for that coin, but this time follow the wire the other direction until you're over the crevice mentioned in the hillside crevice coin. Bounce over the pole to a wire attached to the top and follow it to the end. Bounce three times and over the next pole to get to highest high wire in the level. There is a stack of gold coins with a blue coin on top, either rocket launch or bounce a few times to collect them. Available all episodes.
Sand bird domain (x4)
Follow the walk-through for episode 4 to reach the home of the sand bird. Check the clouds here to find four blue coins and a 1-up. If you get stuck on a cloud or miss a coin, and if you have the patience, you can always wait until the sand bird circles around again. Available episode 4 only.
Hop up to the roof of the Sand Cabana and hover to a small ledge on the left. The are some gold coins, a rocket nozzle crate depending on the episode, and a graffiti M on the wall. Wash the wall for a blue coin. Washing the ground between the cabana and the ledge will make a tree grow, making it slightly easier to get between the roof and the ledge. Available all episodes.
Paired triangles (x2)
Locate the graffiti triangle on a cliff face at the right end of the beach, then locate the corresponding triangle near where the cliff face ends at the left end of the beach. Wash one triangle off to make a blue coin appear at the other triangle, then quickly get to the other triangle to collect the coin before it disappears. It's easier to do this with the turbo attachment but it's also possible to do it with the slip-slid move. It may be useful to clear some of the cataquacks off the beach first. Available all episodes.
Miscellaneous washing (x6)
Sand icon (x4)
There are four spots on the beach where washing the sand reveals a blue shine icon. Keep washing and a blue coin pops out when the icon is complete. Keep watching for sparks in the sand to find most if not all of these, since you will be spraying a lot of water to deal with cataquacks anyway. To get the first one, go straight from the start to the beach, bear left when you reach the shore, then zigzag while spraying to wet a wide swath of sand until you see sparks. For the second one, locate where the shore bends out slightly to the left of the peninsula, in some episodes there is a catamaran parked here. Start washing going straight inland until you see sparks, which shouldn't be far. For the third one, walk down the boardwalk away from the Surf Cabana, bear right when you reach the sand, and start washing from there. The last one is easy to find since it's right in the middle of the peninsula. The first three are available all episodes except episode 3 while the one on the peninsula is available all episodes except 1 and 3.
Blue parrot (x2)
There are two blue parrots in this level that each turn into a blue coin when sprayed. All the birds here will eventually land on a wire above the small stadium, so just wait there and zap any birds that come close until you get all of them. Available all episodes.
If you've found 29 blue coins on this level and are stumped about where the last one is, this is probably it. Find a piece of fruit and carry it to the Surf Cabana. Back-flip to the roof and drop the fruit into the smoothie machine in the center, being careful not to fall in yourself. A blue coin will pop out of the machine. There is plenty of fruit available in the Sand Cabana starting episode 6, but until then you can use one of the coconuts growing in the treetops. The only hint you're given to find this coin is the fate of your watermelon in episode 8. Available in all episodes.
On a beach full of blue cataquacks (so called by the Noki's who sit near the Surf Cabana), there is a single red cataquack. Spray it to knock it out, then hop on top to get a blue coin. Do this to the blue ones to get gold coins. Available episodes 1, 2 and 4.
All the blue cataquacks on the the beach have been replaces by red ones. But there is still one odd colored one, look in the treetop between the wooden platform and the tower to find him sleeping while the others do all the work. Wake him with a squirt of water, then defeat as usual to get the coin. Available starting episode 5.
Yoshi needed (x2)
Yoshi can be found in the middle of the peninsula in episode 6 only. Most fruits can be found in the Sand Cabana, coconuts can be found it a tree top.
Hidden dune bud
Get Yoshi and spray the orange goop in the sand to the left of the Sand Cabana. It will melt away revealing a dune bud. Spray it to create a footprint in the ground and collect the blue coin that's there with some gold coins.
Get Yoshi and destroy the beehive under the wooden platform near the tower, just like the beehives in Bianco Hills.