Surfboard is a Boatitem in the Shipwrecked DLC, and is exclusive to Walani. Walani starts the game with it in her inventory and is the only character able to craft it. It can be crafted with 1 Boards and 2 Seashell from the Nautical Tab, but doing so requires a Science Machine.
It has ×1.5 the sailing speed of a Raft but has ×0.5 times the durability of one, and will last two days. She can hold tools (including Torches) while surfing. A Cargo Boat with a Cloth Sail will have the same speed as the Surfboard but an Armoured Boat will go faster, though.
It gives +1 Sanity each time Walani rides a Wave and ×5 speed boosts from Waves. It will suffer less of a Wetness penalty from hitting Waves, but will not decrease any damage to its durability from impacts with Waves.
Bugs
Fully chopping down a Mangrove Tree while on a Surfboard may cause a crash.