Boats are craftable vehicles that increase movement speed in the ocean biome and also work as portable bridges, as you do not lose oxygen when riding one. They are very handy when doing chrono quests, like the orange gem quest. However, be aware that it's very hard to get one in the queue of the quest.
The controls for the boat are very simple, using your keyboard, steer it in the direction you want it to go with the WASD keys. The sled also has similar controls. To get off it, you need to stop it and click on the slot in the inventory that it is located.
The boat was added in the "Faster" update.