Okay with the talk about lances and melee weapons, I have an idea about Lances. Instead of just a lance, make them Tridents, and give us water mobs! Tridents will only work in water. Maybe have water beast that only aggro if attacked, and have diff levels of them on diff worlds.
Quirks of that sinking feeling and others that boost breathe could give us more to craft, or underwater gear to wear.