Using Epoxy
I have a Hunter 430, 1995 vintage. It has Aluminum water tanks and the forward one under the forward berth started leaking around 2002. I could not for the life of me see a way of removing the tank short of lifting the deck. I cut a couple of inspection hatches in the top of them, due to the baffles they have, and laboriously ground and sanded, then glassed the base and the sides through the small inspection hatches. No leaks since then! Watr was not great initially, but now no problem. Both holding tanks also leak, but I do not use them in my oart of the cruising world in SE Asia.