Aqua Rock is probably the easiest rock out of the other 3 you find. First you must scale the mountain, dodging water flows, and reach the top. You must then use Douse on a rock that will open access to the dungeon. During this entire dungeon, you'll be dealing with water levels and ice, so make sure Piers has enough psynergy to do both. You'll also have a little treat of finding a Mercury Djinn here, however it is unobtainable until you get the psynergy that this mountain gives you; Drought. It is only learn-able by Piers, and can reduce the water levels of small bodies of water. Sorry, but you can't get rid of the Ocean.
This entire dungeon is very peculiar, mainly because it serves to actual purpose to the main storyline, unlike the the Air's Rock and the Gaia Rock and share the same with the Magma Rock. However you do need to get through this dungeon and get the special psynergy it gifts (because without it, you wouldn't be able to enter the Tundra pillar to the south) and that it provide the a Djinn, so without it you cannot fight Dullahan.
As said, this dungeon is fairly easy to navigate through and the monsters aren't that hard to beat. There is no boss to this dungeon. Though, it has been proven to be a little tricky with water levels, but nothing that isn't easily fixed by leaving the room. Not a lot of backtracking, but a fair amount of it.
Though, this dungeon does give you time to level up before heading off to Gaia Rock, as it's been proven to have a lot harder enemies (excluding the boss) in which you should take the advantage of leveling up here and getting the various weapons and armors that this dungeon provides. You'll find some smithing material, so make use of that with Sushi in Yallam and also remember on your way out to get the rusty sword that is under the water that you use Drought to be rid of. Then you'll have an easy way out.
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