TMM offers an option for the Lagoon 42 to add a second water tank in line with the first tank. This increases the water tankage to the advertised amount. The lagoon 450 has two independent tanks selectable via a manifold so that if a guest leaves a tap on, then you don’t run out of water. You just switch to the opposite tank.
Warren S/V Scuba Doo Lagoon 50 (winner of best crewed yacht under 55 feet at the St. Thomas crewed yacht boat show).