So, I tried to kill myself unsuccessfully on the island spawn. I broke my legs soon before hitting the water to swim over. Thinking I would just suffocate, I crawled under the water. I noticed that the Sea had a layer of water and then a deeper layer. When I passed the lower layer, I didn't have to really do anything...didn't get colder, didn't need air, nothing. I eventually made it all the way across to the other side crawling under the sea. I eventually died due to lack of food and a broken leg, but it was funny that nothing happened to me until I hit that upper water again.