The same happens for me. I play in a Realm with 2 other people and the same thing happens to them, too. It's weird: if I leave my boat, the boat disappears and same with anything/anyone who is also in the boat. My two friends, however, can still see the boat and whatever/whoever is still in it. It's the same when they leave their boat; it disappears for that person, but I and the next person can still see the boat. If person A gets out of their boat and person B gets on the boat, person B disappears and reappears only when out of the boat. We all had to eventually craft a lot of boats just to wander around. Exiting then re-entering the Realm fixes it, but it's inconvenient to do every single time we get off a boat.
The same happens for me. I play in a Realm with 2 other people and the same thing happens to them, too. It's weird: if I leave my boat, the boat disappears and same with anything/anyone who is also in the boat. My two friends, however, can still see the boat and whatever/whoever is still in it. It's the same when they leave their boat; it disappears for that person, but I and the next person can still see the boat. If person A gets out of their boat and person B gets on the boat, person B disappears and reappears only when out of the boat. We all had to eventually craft a lot of boats just to wander around. Exiting then re-entering the Realm fixes it, but it's inconvenient to do every single time we get off a boat.