If it's a new tank, it guaranteed not to leak, they screwed up when they welded it, probably a porous weld. My 19 yr old gas tanks have no leaks whatsoever.
That's why everything is supposed to be tested and quality assured, nonetheless, and coating you use will eventually loose adhesion, don't settle for coatings - only as temp repair, but don't do it yourself.
Don't agree to anything.
They should send you and the boat to a shop prepaid and have a another new tank installed, and do it w/o delay, and with thier apologies.
Don't attempt any repairs yourself to protect youself, the tank is dangerous as is and their responsibilty unless you damaged it upon installation.
Discuss with your lawyer who can send out a letter, means a lot at other end. You would be surprised.
Good luck.