The first thing to check is if this is a real connection; I suspect not.
Put a 12V or 24V light bulb (technically called a lamp), across the meter terminals and repeat the measurements. Don't use an LED or a small indicator lamp, 5W or more is needed.
The lamp will cause a higher current to flow, which will drop the voltage if the resistance is high. If the lamp stays lit, there is a connection. This is always a good method to use when checking stray voltages.
To stop you worrying, note that my keel is also connected to negative, via the VHF, through the antenna bracket, the mast, then the lightning strap. Took me ages to figure this out. I'm considering making a proper connection, as there isn't much I can do about it.