I have found that when you leave a boat, the boat disappears.

I only know of this happening to me, but I'm sure it is a bug that affects multiple others. The boat that you exit from in a world disappears instantly. Saving and exiting the world seems to make the boat reappear. It is very similar to the bug that causes a horse to disappear after dismounting it. This bug seems to have accompanied the 1.18.0 update to Minecraft.