All water levels from Sonic 1, 2 and 3

4-3 from Felix the Cat (NES) - mostly the reason I like this level so much is that the music is really dark, which contrasts heavily with the rest of the game's music (it's very happy go lucky). The level itself is pretty good as far as water levels go, but the dissonance created by the music really made it memorable for me.

7-4 from Mario 3 - This was a difficult water level that wasn't unfair. I always died a couple times, but beat it after remembering what to do.

I second that one. This level made my head explode.

I'M going real old skool here...

And all the Water levels in SMB1 remake. Not only are they fun, the tune is pleasant to hear.

The Sunken Ghost Ship Level in Super Mario World is an all time favorite for me as well.

And finally the Dolphin Shoals in Mario Kart 8.

Any level from Super Mario Sunshine

...none of them comes to mind :/

I usually don't like water levels, but I did enjoy Dire, Dire docks in Super Mario 64 and 20,000 Lums under the sea in Rayman Legends.

That would be my pick! Everything fits perfectly and dat Boss.

Aqua Rock from Golden Sun Lost Age