I can confirm that the following vehicle characteristics are built into SA-MP and cannot be suppressed by server developers.
When a vehicle without a driver enters water anywhere on the map, it will respawn very soon. This period appears to be about 10 seconds. Therefore, even vehicles with a passenger will follow this behaviour.
However, this feature is perfectly viable. We can simulate with relative realism, a sinking person under the condition that they are tied and in one of the game's oceans whilst barely having an impact on the server's resource usage. You may well see this, or a manifestation of it, in a future update.
Thanks for the suggestion!