Well it can be like how sex works on the floor.I don't think it's doable unless there's some rigged object in the water.
The good news is at least sex works on the wading parts of the water so far. But it is like lly we will have to wait until turnodrivet comes back before we can see rigging for the water.
Worst case scenario is you ask a modmaker to try and see if they can do it or not. Modders like Anarcis do take requests after all