Does anyone know how to get the treasure chest that is across a pool of water in the dungeon? I've searched everywhere for hidden/bombable walls but still can't find a way to get it.

Once you've unlocked the "water way", you can go back to grab this chest.
That's the first dungeon, right ?

Is this the same chest behind the pool of water with a single submerged platform? I have problem with this chest too. I already have bombs.

You should try to travel the whirlwinds. I must admit, I discovered that by accident.

The whirlwinds throws the player into a bottomless pit (unless one of them is an exception).

Got it, but it looks like losing health here in unavoidable.