I believe they are not Coast Guard Approved. You may be able to get insurance as most of the time you just need to provide pictures of the boat, engine and interior. If you have to file a claim, especially if it was fuel related, may be an issue?
Ya, any opening in the tank must be on the topmost surface. Sight tubes and drains/gravity feed outlets aren’t USCG approved. I had some tanks made for a boat that are gravity feed and I’m inclined to leave them that way if there don’t seem to be any insurability issues.
It's not so much the insurance I'd worry about... it's the potential BOMB / FIRE BALL that a massive leak could lead to. In an enclosed space I'd say NO WAY. In a more race style deal it's probably not as likely to kill you as you'll almost immediately smell and see the gas pouring into the hull. A buddy you know developed a crack (or hole) in a tank and that turned into an OH-CRAP real fast. It would have been a massive fire if a source of ignition was present.
Now delete this thread so the INS CO doesn't get wise to ya!