Oceans are full of water, islands, underwater caves and sea animals. You can find lots of creatures in the oceans such as Piranhas, Sharks, and Seahorses.
It is highly recommended to use a boat (which means at least 1 point in sailing skill) for traversing an ocean biome. Swimming across one is entirely possible, but the default swim speed is roughly half of a character's walk speed, making this an excruciatingly slow prospect.
While traversing the underwater caves, the player can quickly rise back up the surface by equipping the boat while underwater. This can be used to allow for deeper/longer dives beneath the ocean.
As of the Cube World Steam release, the player now needs to obtain the boat from a mission instead of investing a skill point into your sailing skill.