A Sukkah on a ship or car:[1]
A Sukkah which is built on ship or car is valid so long as it [i.e. the wall and Sechach[2]] is strong enough to withstand a common wind on dry land. If it is sturdy enough to withstand the wind on dry land, then it is valid even if it cannot stand in the face of a common wind of the sea. The Sukkah remains valid even when the ship or car is moving.
[1] Admur 628:4; Michaber 628:4; Mishneh Sukkah 22b
[2] See Q&A!