I've been finding that every time I get in a boat and travel far enough that the location I got into said boat is out of simulation distance, the boat teleports back to that location. I did some testing and found that anytime I leave a boat, it teleports back to where I got into it and then quickly warps directly towards me, but if it teleports to unloaded chunks, it won't warp, leaving me with no boat until I leave and rejoin the game.