If can remove the tank from the boat, cut large access holes into the top, cut the entire bottom off and do a major repair/replacement of the bottom, then (through the large cuts of access) apply a FDA/NSP "for potable water service " rated epoxy liner. Lining epoxies are 'rolled-on' and are special formulations that emit very low amine content when cured.
The large access holes can 'studded' using common 18-8 bolts to which to apply large plates (drilled to accept the studs added to the tank), and sealed with thick Silicone caulk (which is rated for 'potable water service'.
This will result in a cost of approximately 1/2 the cost of a new tank and if 'lined' with a FDA type potable water epoxy will last indefinitly. This is the way that potable water tanks aare repaired by airlines, railroads, commercial shipping.
Aluminum tanks that are charged with water that is chlorinated will always 'rot out' due to the formation of aluminum chloride. The chlorine in the 'chlorinated water' aggressively attacks the aluminum. There is no economical way to sanitize an aluminum water tank. Epoxy coatings ("Lining" epoxies NOT structural epoxies) are the solution for aluminum tanks.
Most 'commercial divisions' of marine coatings manufacturers have "potable water" lining epoxies. The best supplier for small quantities (quart quanties) Ive found is: NSP 120 - NSF 61 Potable water approved Epoxy Adduct