Aqua Rock is the second in the series of long and somewhat tedious elemental rocks, although thankfully it's not nearly as long as Air's Rock is (although the inside is somewhat harder to navigate). Follow the waterlogged path up to the base of the rock. Climb up the first wall, and then take the one on the left. For now the right wall is unclimbable because the gap between the falling water isn't long enough. So, climb to the left. When you reach the pillar, Move it along (you might have to climb around to the other side of the first moai) until it blocks off the second moai (the climbable wall here is a dead end). Now you can scale the right wall. Go up the next wall to the area with the deep water, but go down the ladder to the right and go off the waterfall. Down here, be careful not to accidenally fall off again or you won't be able to get the chest (which contains a Nut).
But you have to fall off anyway. The rocks will catch you, so get off the watefall to the right. A pillar blocks the wall to the right, so climb down the ladder and use Douse on the moai here to make it create a watefall. Drop down using the watefall to the left (but only one level; this is important) so you can use the new waterfall to drop down onto a previously inaccessible area.
Follow this path around until you reach a fork. Take the right-hand path and use Douse on the blue rock (if you haven't set Douse to a Psynergy Shortcut, do it now). This will make the watefall above it reverse direction (impressive). Now go back to the fork and take the left-hand path. Going up the left-hand side of the wall will net you an Elixir, and then you can go up the right and keep climbing up. You can go left at one point and move the pillar that was blocking you earlier onto a metal plate to open up a shortcut. Now, head over to the waterfall and let it carry you upwards to a new area.
Go up, so you can get across the waterfall and get off on the right-hand side. Move both the rocks onto their respective metal plates, and you can get across to an Oil Drop. The wall here leads up to near the summit of Aqua Rock. At the top of the wall, don't take the next ladder but use Frost on the puddle up the ladder to the left, and also on the two puddles above that. Now you can go back around and weave between the rocks. Make sure you go across the two ice pillars at the top because the chest on the other side contains the Mist Sabre. Felix should equip this, and give Piers the Disk Axe if the weapon he currently has isn't stronger.
Hop across the lower ice pillar now and climb down the wall. Go left to the next area. Use Douse on the big blue rock to create a whirlpool in the middle of the deep pool below you. Hop across the platforms and jump into the whirlpool to be carried down into the interior of Aqua Rock.
Aqua Rock Interior
Head down and into the next room. You can't go anywhere else for the mean time, so just head across the platforms and through the door on the right. Head through the next door and you'll come to another pillar/airjet puzzle. In order to get past, use the pillars to block the airjets, then on the other side use Move. You can set them up so you can jump around the rock and get a Vial from the chest. When you've got it, go back down the ladder and leave the room out the bottom exit.
In here, move the pillar to block the moai and drain the pool. Freeze all the puddles and use the ice pillars to jump across and exit the room to the right. Head down and to the left along the bottom path to get a Crystal Powder from the chest, then use the north path to re-enter the first room. Hop across and use Douse on the blue rock to form a path out of the room to the top left. In the next room, go to the left, and follow the path around to the airjets. Block off the right-hand one with the pillar using Move, then go up that path into a new room.
Before you use Douse on the statue, rearrange the pillars so they form a path (the lines on the top of the pillars indicate this) to the ledge on the right. This first puzzle is easy - move the top right pillar once to the right and once downwards. Then use Douse on the statue to fill the room with water, then again on the blue rock to create the path across. Hop to the next stairs, and keep going down until you reach a purple room. This is very similar to the one at Air's Rock, isn't it? You can't reach the infinite Psynergy Stone here yet, so just head up the stairs to the left. Follow the long path until you reach a large room with a river.
Get on the river at the very top here, and run to the left (make sure you try to run towards the upper left, so you're fighting against the current and slowing your movement down). Before going down the ladder, run onto the river again in the small area surrounded by rocks, so you drift down and can use Move on the pillar. This will allow you to get a chest containing a Tear Stone. Use the groove to slide back down, and take the stairs down to the right.
This room is a bit like the entrance hall for Aqua Rock - it leads outside, to the "heart" and to various other places. Until you get the Parch Psynergy (from the tablet at the "heart", which is unreachable until you get a certain gem) you can't do anything through the top right door, so go to through the top left one. In the next room, climb down the ladder, use Douse on the statue, and hop around to the door at the top. You can't get around the pillar up ahead yet, so go around to the left-hand bottom door. In the next room, climb down the ladder, move the rock one space to the left and use Frost on the ice pillar. Go back up and use Douse on the statue, then you can hop across to the chest containing a Water of Life (which is very useful). Leave the room and re-enter to drain the pool again, then shove the rock two squares to the left, then use Douse and hop to the exit.
Back in this room, Move the pillar into its hole and go up to the top door. Move the pillar right and then up. Climb up and use the pillar to hop across through the top door. Go across the tightrope and along the path to the next room. Rearrange the pillars in here so that they form a path across, then use Douse on the statue and blue rock as before. Down the stairs, shove the pillar off and jump across to the door. Hop your way up to the top of the next room (the chest to the right is a Mimic) and then head out the left door. You'll be back in the purple room - you can now reach the infinite Psynergy Stone. Use it if you need to, then head down.
Getting the Aquarius Stone
Use Douse on the blue rock to form two paths across the water. Hop to the left to get a Lucky Pepper from the chest, then use the right path to get to the rock in the middle. Climb up and take the Aquarius Stone from the chest. Head over to the right, and take the ladders down to the bottom of the room (take the left-hand door when you get there). Back in this room, you can go through the middle door. Place the Aquarius Stone on the pedestal and it'll part the water to form a path to the "heart".
You'll be on platforms on an endless sea. Hop across, and climb around to the tablet.
"Wielder of Water's strength...Lay your hands upon this stone. If th'art worthy, the power to drain away the standing water shall be yours."
Piers will learn Parch! As soon as you do, water will fill the pool you climbed through before. You're not trapped - use your new power to drain the water away so you can get back to the door. You can now leave Aqua Rock the proper way, but there's still something left to get. Steam, the Mercury Djinni, can be yours by going right in the main room (where you went through to get to the Aquarius Stone's slot and part the waterfall). Keep going (you have to climb up the right-hand side of the large room) until you reach another long river. You need to get across to the other side, so make sure you always run towards the top right so you're fighting the current the whole way, and on the other side head all the way up to the moai statue spitting out water. Use Parch on it to stop the water, and now you can use the ladder to the top part of the river to walk up the channel and confront Steam.
You will kick yourself if it runs, since you will have to stop the water again. For this reason, it's best to simply standby every single Djinn you have and summon rush it on the first turn, and hope its either not moving afterwards or you're still strong enough to resist its powerful Mercury Psynergies. Once you've got it, head back to the main room - you can now leave Aqua Rock the proper way.
Head back down until you're back in the entrance room, and in here use Parch on the water. You can now not only leave Aqua Rock, but get a Rusty Sword from the chest on your way. Climb down the ladder to the right, grab the chest, and climb up the ladder in the middle and exit via the door at the bottom. Use Retreat when outside, and leave Aqua Rock.
Now, you will want to use an Inn (and possibly a Sanctum as well, or Felix's Revive - do NOT use your Water of Life), so head back to the Apojii Islands and use that one. You can also stop off at Yallam to forge what you've collected if you have the money, although you can make a visit once you've come back from your next destination, since it's on the route. Your next destination is the tower on the west side of Tundaria - but because the only beach is on the eastern side, you will have to make a very long and very tedious journey across the cold, southern continent of Tundaria to reach it.
Wheeze, the Jupiter Djinni
Before you actually enter Tundaria Tower, make sure you go to the southwest. Go as far southwest as you can, and walk around in between the two odd-coloured ice patches. Eventually, Wheeze the Jupiter Djinni will appear. It is quite fond of running, so really pummel it with attack Djinn or strong Psynergies (overkill it with summons if you want). Once it's down, it'll join your party (set it to Sheba). Head back northeast and enter the blue tower that is Tundaria Tower.