Hi.
How can I detect that the actor is under water?

since the water is just a plane, and you know the position of that plane, i guess you can just check the position of the actor and look if it's lower than the position of the water plane

not exactly since there can be many lakes (many planes), and also rivers which can have different height levels.

but no worries, in the next engine version there will be a function for detecting if position is underwater

Ok