I was just doing this in my game and noticed a couple of things. The boars only seem to wander and have problems clipping when close to land that they try to move towards. When i moved two boars on my longship around an island as long as i was far enough away from shore, they stayed put.

Your other option which is slower but safer, is to turn pvp mode on, harpoon a boar and stay tethered to it while you make the trip, make sure and get it on the deck though, or it can push the boat around etc.