Super Mario Galaxy 2/Cosmic Cove Galaxy

Twin Falls Hideaway
You start out on a beach and need to head into the water to get to the main area through a short underwater tunnel. You can grab a green shell, either found underwater or obtained by stomping the Koopa Troopa on the beach, and it will allow you to swim faster and gives you a light you can easily destroy the Snoodles with. Talk to the penguins in the water for some tips on controls in the water. You can only stay underwater for a certain length of time, shown by a green air gauge that slowly depletes. Make sure you run into air bubbles or come up for air so you don't lose any health.

When you get out to the main open water area, you'll find a lot stuff to do from mini Launch Stars to a ? Coin. Off to the side is a green warp pipe with three Chance Cubes inside and a 1-Up Mushroom in a crate underwater behind the stone tower at the far end. There is also a Hungry Luma on the wooden platform next to the stone tower and it wants 15 coins to make a new planet for the hidden star. When you're done exploring, ground pound the red button on top of the stone tower to freeze the planet, then head over to the twin waterfalls and wall jump up them. At the top, break open the crystal on the left to free the Power Star, then jump over to the other side to get it.

Exploring the Cosmic Cavern
This time the water is already frozen over and you'll need to find a key to free the Luma trapped on the beach. Head out onto the ice and avoid the Li'l Brrs to find the green warp pipe at the end. Take the pipe down underground and ground pound the bricks to continue. Once you're in the water, swim to the other side and get out. Here you'll find a hill with water globules rolling down it. If you get hit by the water, you'll roll all the way back down until you crash. Head up the hill jumping over the water and spinning, then wall jump up to the next area and head right over the spiky plant. Grab the checkpoint then head back into the water and swim downward. You'll go down past some spiky plants and penguins until you hit an air vent that will push you over to the right where you can spin a switch to open a grate allowing you to go farther down. This tunnel is full of Gringills that pop out of the walls at you, but if you just keep heading down and spinning you'll be past them before they can strike. At the bottom of the shaft you'll find another green pipe.
 * First planet

You come out of the pipe up in space on some orange and red striped platforms. Head to the right and grab the checkpoint flag then jump up into the water. Swim around the circle avoiding the Jammyfish and continue out the top. Continuing to the right, avoid the first mine and optionally pick up the 1-Up Mushroom beneath it. Next, swim up between the two mines on the right and jump out of the water onto the platform by the Paragoomba at the top. Jump into the water beyond it and swim up the the striped platform with the penguin on it. Here you'll need to keep in the center of the moving water platforms without falling out. When two of them come together you can swim between them. Jump into the first one and switch to the next one on the right, then a smaller one going up. At the top spin to jump out of the water and grab the key freeing the Luma. Jump onto the platform up there and take the green pipe at the end back down to the beach. Talk to the Luma and it will transform into a Launch Star for you. Grab the checkpoint on the Toad Ship and launch to the final planet.
 * Second planet

Here's you'll be confined to one side of the planet full of Fizzlits. There is a Spin Drill here and some soft patch of dirt you can drill in. Most of the patches just hit the stone of the other side of the planet and bounce you back, but one contains a 1-Up and another leads to the open patch on the other side. You need to drill in the center of the long strip of dirt next to where you picked up the Spin Drill to get to the other side. Over there you'll find a bunch of Mecha Mini-Bowsers you can jump and drill over to destroy. One of them has the key to unlock a water hole you can drill into to flood the planet. From there, simply swim over to the wooden platform to claim your Power Star. When you finish this level, a penguin will appear on Starship Mario.
 * Third planet

Catch That Star Bunny (Hidden Star)
In the big open water area, feed the Hungry Luma next to the stone tower 15 coins to transform. Take the pink Launch Star up to the new planet, where you'll find it's covered in water and Skeeters and it's full of holes. Talk to the white Star Bunny to start the mission. First you'll want to go ground pound the blue button next to the checkpoint flag to turn the planet to ice, then spin and skate around chasing the Star Bunny. This is more of a skill star rather than strategy, so just follow the Star Bunny and try not to fall into any holes. If you don't catch the Star Bunny before the world unfreezes, you'll need to go back and stomp the button again. Time can run out even after you've caught him, so be careful on your way to the Power Star too if the enemies are unfrozen.

Green Stars
This Green Star is at the top of the twin waterfalls, so freeze the planet with the button on top of the stone tower and wall jump up the waterfalls. When you get up there, run to the back of the white fence behind the crystal with the Power Star in it, and you'll see the Green Star there. Jump over the fence to grab it.
 * Green Star 1

Both of these Green Stars are above the planet in the area with the striped platforms when you get through the swimming portion. In part with the two mines going up and down side by side, go out into the small square of water on the right side with the coin in it and jump off to the right from the top with a spin to fall down onto Green Star 2. The third Green Star can be found by long jumping off the left side of the platform where you get the key to free the Luma.
 * Green Star 2 or 3