Boat water tanks that are made of stainless steel are CRAP and are easily subject to corrosion unless the following (expensive) steps are taken: the internal surfaces must be free of 'weld irregularities' and the wleld should be ground flush and smooth, the internal surface finish (including the weld areas) should be sanded and a high mechanical mirror polish (to ~5 micro inches) followed by 'electropolishing' to get any 'service life' out of tank that receives 'chlorinated' water. ..... all this is entirely too expensive for a boat owner. Boat builders dont do this because its also toooooo expensive to do.
Repair welding that doesnt include the above will result in such a tank immediately subject to 'halide' (from the chlorine in 'city' water) corrosion especially in any 'heat affected zone' from welding etc.
Such tanks can 'easily' and economically be repaired by overlaying the previous welded seams (usually where the leaks will be found) with fiberglass cloth tape, a good strength epoxy and then ....... 'lining' the entire interior surface with a low amine emitting, low 'leachables' epoxy especially formulated for food grade or potable water service. These are FDA rated epoxies for 'potable water service', are applied with a paint roller (just like you apply barrier coat). All you have to do is clean the tank, roughen the internal surface with a grinder/sander, apply the strength epoxy to fill-in the pin holes, apply the cloth and then roll-on the potable rated surface. Most of the boat paint manufacturers offer 'potable water' rated lining epoxies though their 'commerical marine' divisions. A good source for small quantities would be: NSP 120 - NSF 61 Potable water approved Epoxy Adduct
(good product, lousy website). You can do most of this work if you have large access ports in the top of the tank or cut new access ports .... can be done in the boat or you can remove the tank and do it in your 'shop'.
You can rebuild most metal tanks (fuel, water, etc.) in this manner at a ***fraction*** of the cost of a new tank.