Currently, the Ocean has no threats. Drowning is impossible, and currently Sharks, while they exist ingame as an item, do not attack the player. Also, Pirates do not sail in the Ocean as of yet, however the feature has been hinted to exist in a future update.
Swimming is a basic feature in Salt. It automatically occurs once a player enters water that is deeper than the player's height. While swimming, a player's stamina gradually decreases. However, there is no consequence of drowning once the player's stamina reaches 0. As of right now, there are no items in the game that increase swimming speed or ability.
While it is possible to go Underwater, either by jumping in or pressing Ctrl while swimming, it is impossible to see underwater as a graphical overlay is placed over the player's vision. This is likely a placeholder until underwater environments are created for a future update. Also, there is no indicator for Breath, nor can you drown.
Shallow Water is water surrounding all Islands, and is typically characterized by calm waves and a lack of music. Only a limited number of fishes can be caught in this type of water.
Players can escape into shallow water if they encounter a threat on a particular island that they are unable to handle. Any mob following the player will run away once the player enters the threshold of water. This can also be exploited to pick off tough mobs with a Bow, as they'll constantly run back and fourth if the player enters and leaves the water.
Deep Water is water that exists far away from islands, and is signified by higher, more violent waves, and sometimes music will play. Despite the existence of waves, it is not yet possible for a boat to be sunk as a result of waves.
While currently only serving as a means of transport, the Developers have hinted at the addition of many features for the Ocean to come in later updates. These range from sailing pirates and marauders, sea monsters, and underwater features.