Running out of fuel never happens to me except due to battle damage (by the time I leak fuel, diesel is the least of my worries, shipping a boatfull of water more likely is
).
The real problem that would be cool to fix would be the inability to move on with a career EVER if you are stranded. While I am unaware of boats being towed, I know for a fact a few were rescued by other subs (crew taken off, boat scuttled). I THINK I read of one having some fuel transferred, though it didn't work well.
I have been sunk in game at least once where I miraculously was given a new boat, so the game clearly supports the ability to rescue you when sunk some random % of the time. Wish we could pry into that bit of code, and have the ability to call for help or abandon ship and get rescued if we are within certain areas of the map...