Getting out of a boat will sometimes make it disappear for the player that was riding it. Traveling out of simulation distance, then traveling back to where the boat should be allows the original rider to see/interact with the boat again.

We just started a new world for 1.18, and people are losing their boats. This is frustrating and can be dangerous depending on where the player is. Also I have noticed a few boats around already that appear to be abandoned since the player was unable to break or re-enter they're boats. If a player doesn't have multiple boats, and they stop a at small island to sleep, they can't find their boat or make a new one, so they are stuck swimming.