Hi,

I'd like to ask, is there some way to interact with water, eg.: putting some waterboat on it, adding splashes and "tail" behind a sailing ship? I'd like to do it both on default water (infinite) and on some world-made river/lakes.

you can simulate boat physics manually
boat splashes simulate with particles
on water surface effects will be in the future

About boat physics:
Is it possible to cut some shape (mesh, phys or actor) with a plane and calculate, which part is over and which is under the plane (in a volume percentage, for example)? Generally, I'd like to know, how much some shape is immersed in water. Is it possible to do with actual physics engine?

just calculate the min.y and max.y of a shape compare them to water level and take some average

Is it possible to toggle through position of all vertexes of an actor? (for computing min/max y)

that would be inefficient
you can use Actor::shape if its from 1 shape
or secondary Mesh::box transformed by Actor::matrix

you can also use Actor::massCenter..

OK, I've found quite nice and effective algorithm of calculating buoyancy forces in water. I need to treat water as a plane (which is already done in engine, I think) and if some object collides with the water, I need to make bounding box of the immersed part. Then I calculate volume of the bbox and apply force in the middle of it.
My question is, how to calculate to bbox of part of an object, intersected by a water plane (red part):