For me monel is the ONLY tank metal suitable for a boat - if you''re near a ship ''breaker'' /salvage yard sometimes you can find fairly economical monel/inconel. Otherwise I prefer Fiberglass with an
epoxy liner followed by good old black iron.
Yup, black iron is cheap, strong, everyone and their brother knows how to weld it. It will last forever if you recirculate/polish the
fuel regularly and draw it through a water ''knock out pot'' followed by a ''starch'' (hydroxymethlycellulose) impregnated filter. The knock out pot removes all the free water and the starch removes the emulsified water - simple, cheap and very effective.
..... just make sure that you ''blue'' (ferric- to ferrous) the exterior bottom so it doesnt corrode externally ... and then swab occasionally with phosphoric acid to keep the ''blueing'' intact.