When exiting a boat it will both disappear visually and physically from that players view. Other players in the same game will still be able to see and interact with said boat. If any of those player (or any entity) interacts with the boat they too disappears visually and physically. Although any player/entity in that boat is still able to interact with the world. Said boat will reappear if the chunk it’s in becomes reloaded or another player breakers said boat.