In practice, you may have some distance and subcompartments between the low bilge and the propane "box".

As others have said: The propane tank should be SEALED except for a "drain" which is open at the bottom of the tank and will (without traps) open to the air low (but above the water).

You still have the issues of a leak between the "box" and the stove. You should PERIODICALLY check to see if you maintain pressure by turning the valve off to the tank, then seeing if the pressure holds steady for (say) overnight.

If you don't do so, then you might simply not have to worry about this, the propane system will remove itself on not-so-fine day or night. :^))).

(P.S. - if you have a system which was properly installed with padding where the hose goes through bulkheads partitions, etc. and you don't mechanically damage the lines. The system should last a LONG TIME. Just check it periodically. ANY signe of pressure loss should be assumed to be a problem. I know it sounds stupid to say, but way too many plumbers and homeowners don't remember so: DON'T CHECK FOR GAS LEAKS WITH A MATCH! use soap and water, your nose, or a special "sniffer" device.)