Water

Water is a common topic in Dual universe. Lush planets like Alioth certainly contain liquid water on the surface, as can be seen from space. A trade-off will need to be made between realism and performance. At best, this will result in a local voxel-based flow system, that behaves similar to the procedural generation in that it renders for players based on their proximity. More complex fluid mechanics such as full fluid system with multi-scale management are unlikely.

There is an underground water table on planets such as Sanctuary. At this time it seems that the water does not respect any flow though and simply fills any space that is mined underneath the water table.

Being underwater does not impact the player's movement speed, nor can you swim in it; falling through water is like falling through air. However, hover engines and vertical boosters can still interact with it. Being underwater does have different ambient sounds though than being above water.

Trivia

 * Since hover engines and vertical boosters interact with water, you can use them to launch yourself into the air by sinking to the bottom and turning them on again.