Castlevania: Symphony of the Night - Part 04: Underground Caverns

A water level that isn't that annoying. A rarity.
The only thing slightly annoying is that the water initially damages you, and that there's a slow boat trip over a section of it.
The boss is pretty cool actually, I like the raising water section before it, and the fact it takes up two screens to fight on.
Once you get the Merman (which is actually a Mermaid) Statue you can break the ice on a screen I show at 29:22.
I found this by accident initially and died. It's worth taking this path for the Snorkel. As you'll be able to move without being damaged by water, and explore most of this region for now.