I believe energy actually removed from the source but not radiated out (as radio waves) will be radiated out as heat. Either the unit would heat up or else the antenna would - depending on where the problem (the resistance) is located.
Energy is never lost
so if it is actually removed from the battery
(amp meter) then it must go somewhere. If not radio waves, then I think heat.
You should be able to run a comparative test by deploying another antenna of known quality on a short piece of vhf cable. This will also give you an emergency
/ spare antenna / cable for your boat.
If your boat is visible from any landmark remote
enough, you can then make a real life test with a handheld unit and a friendly soul operating at one end.
Let us know what ensues.