The Thinner Ocean is a sea composed entirely of Thinner that surrounds the Wasteland, presumably created by the Thinner Disaster. It is very likely that before the Thinner Disaster, there was either a "normal" water ocean or a paint ocean, which was replaced by the Thinner Ocean. The color of the Thinner Ocean has varied throughout the concepts; in some early drafts, it was even not green like normal thinner (presumably because the developers had not decided yet that the thinner would be green in the final version). Some concepts depicted Tomorrow City as an area built upon a massive boat travelling on the Thinner Ocean, reminiscent of "Noah's Ark."
Other concepts featured sea-beetleworx, and showed Dark Beauty Castle and the Jug as being located on two very small islands of the Thinner Ocean. This concepts, and two others, also featured the Jug Pump, a complex machinery built by Oswald and the Mad Doctor soon after the Thinner Disaster in order to suck the entire Thinner Ocean into the Jug.
In early drafts, the Thinner was not obligatorly mortal. In these versions, Ursula (or a probably rejected design for her) was supposed to live inside of the Jug (full of Thinner).
In the current version, it seems that only mechanical creatures (such as the animatronic version Tick-Tock the Crocodile) or inert objects like the Jolly Roger can be in contact with the Thinner Ocean (if the sea seen in the Tortooga level is actually part of the Thinner Ocean, which remains unconfirmed).