Would make a nice freighter, or tanker hull, as is, for 15mm, or 20mm modern pirate games on the high seas.
Lots of action about that in the news lately.
If you painted it dark gray, or black, and added some minimagnets at the rear, to attach a superstructure to, you could remove it when not needed, so you could still have your submarine.
Do the same on the bottom of the ship/top of the sub, for the conning tower.