The fuel line from the regulator (or solenoid valve) to the stove must be continuous–no connections inside the boat except at the stove. Solid copper fuel lines are commonly used, but flexible, high-pressure hose is probably safer for boat use, and it is easier to install.
A threaded adapter can be required to connect the hose to the regulator or solenoid. Use thread sealant on the regulator end of the adapter but not on the end that the hose attaches to. Pipe fittings require gas tape or sealant, but flare and compression fittings do not.
The hose should exit the propane locker near the top, and it should pass through a vapor-tight fitting to prevent gas from leaking out of the locker into the interior of the boat. Use a similar fitting to protect the hose from abrasion everywhere it passes through a bulkhead.
Common straight-through fittings are too short to pass through a plywood bulkhead. You can thin the bulkhead by counter boring one side with a spade bit. Alternatively, cut an oversize hole through the bulkhead with a hole saw, screw a metal or fiberglass cover plate over the hole, and install the fitting through the cover plate.
Be sure to protect the hose from abrasion and excessive heat (140°F). LPG hose should run through the top of lockers rather than through their bottoms. Support the hose every 18 inches with nylon cable clamps.