Composite gasoline and water tanks under bridge deck.
I got tired of a portable fuel tank being in the way and always too small, and of not having water for a shower in the summer. They hang from an integral flange with no mounting hardware other than a ring of bolts. Each holds ~ 12 gallons.
And the nice thing about multihulls - no risk of fumes in enclosed spaces.
15 years and counting. The gas tank resin is e-10 rated and the water tank resin is potable water rated.
Probably $50 each.