When using a boat, the positions of the player and of the boat will be desynchronized.
Usually, the speed of the boat client-side will be higher than server-side. When arriving to a specific zone:
→ The player, when leaving his boat, will appear in the sea. The distance between the two desynchronized positions depends of the distance travelled.
→ Server-side, the boat is next to the player after he left it. But the player will be unable to see it.
→ A ghost boat remains on the location where the player has left the boat. It is not possible to interact with it.
Disconnecting/reconnecting solves the problem; the player and the boat will re-appear to the server-side positions.
Linked issues
duplicates
Comments

Duplicate of MC-2931 - If you have not, please use the search function in the future, to see if your bug has already been submitted. If you could not find the original report, please comment with the keywords you searched for.
confirmed, also comfirmable when looking at sethblings UHC Season 16 Episode 1