1. Have good level design that makes the water level not boring. Water levels should be short (they mustn't be very short, but don't make it too long, because long water levels are boring water levels)
2. Please, don't spam Torpedo Teds and the Rip Van Fish. Running away from a fish that is chasing you and from a gauntlet of Torpedo Teds can be annoying. And don't put Torpedo Teds in narrow places.
3. Don't make puzzles in water levels. It's annoying and makes the level more boring.
4. If you want a long water level then make it a beach level with just some water.
5. Plan about putting enemies in water levels. Also don't make a water level laggy because the jump button will sometimes not work (especially Turbo A/B).
6. If you want some custom sprites in a water level, use only these which match a water level.
7. An interesting ExGFX and/or pallete would be nice for a water level.