Basically you're looking for a coordinate space transform for the occupants of the boat. The boat moves in world slave while the occupants move in boat space. General theory answer would be to give the boat object an array of pawns and call the array "passengers", and either set the boat as the owner of each of those…
haha! I was hoping someone else would suggest that. Strad: What class they are totally and completely depends on what you are doing with them and when. InterpActors look to be a replacement for the old Movers of UT99-2004 era, which were essentially level geometry only. I'm not 100% sure HOW a boat would fare being made up…