So after a lot of screwing around i found out that buoyancy requires not just any collision but Convex Collision to work at all. So i thought that was a major victory but then i discovered something very odd. I know its experimental and all but i find this very strange behavior.
I set my boat to 5000kg and it has water friction enabled. When i run into the boat i can push it around a tiny bit, almost like you would expect but when i get into the boat, if i run up against the railings it causes the boat to move in that direction as if its weight free. I didnt think it would ever be possible to push an object from inside itself… All this is, is the buoyancy example BP with a boat mesh instead of the cube, The Pontoons are spread out and enlarged a bit and the buoyancy of them is increased to make up for the 5000kg. There is no logic to this object at all. So if you use that BP and put a UCX collision boat model and get on the deck and run against the rail, it should do the exact same thing to you.