This issue was closed by Geez because he could not reproduce it but it is still an issue.
When you drive a V3S to the prison Island and get out, the truck will be at the bottom of the water. But if the server restarts and the truck does not get reset to a spawn point, it will be floating on top of the water. If you swim out to it and get too close it will freak out and then sink to the bottom again. This does not always happen, but it happens often enough.