It would be the same as an automatic in a car, only simpler! Fluid for fwd would go 1 way to cause rotation, when you shift reverse fluid goes opposite direction.

 

I think it depends on the transmission. If it's a Hurth, which is common, a company called ZM has posted manuals with exploded parts diagrams. My westerbeke engine manual has transmission diagrams, and your engine mfgr's might also.
The Hurth HWB100 I now have apart is very simple. The engine drives 2 gears; they turn in opposite directions. The shaft that drives the prop has a gear that shifts between the two, moving the prop in one direction or the other.
Parts aren't cheap, in case you were wondering.