Kill a sea turtle or tortoise to obtain the shell. It can be equipped as a weapon and left clicking will bash enemies away. Use it as a shield by right clicking. Inventory holds a maximum of 1.
The turtle shell can also be used to construct a water collector.
The shell can also be used as a sled by jumping and doing a downward attack. (
there is currently no fall damage, so you can sled into the sinkhole without taking damage, for example)