If you can get the tank out a stainless kitchen equipment fabricator/Hood builder could weld on a new bottom. Just make a pan 16ga. S/S 304 grade is ok, but 316 would be better, to fit the bottom and come up all sides a few inches and weld it around. Perhaps it can be lifted and welded in place on the boat. Yes, TIG welding or Heliarc welding would be they way to go. It may only need that , clean up the spot hammer it shut at feed the gap with a thick TIG stainless rod in place on the boat. You can use those brazing / welding pads that good AC/Refrigeration mechanics use to aid with protecting the boat from heat/damage. And/or a welding blanket. But if it can not come out I would drain the tank, let it dry . Try to lift the tank at the bad corner if you can even an inch will help, sand it down and seal it with marine tex or 5200 .