Deep Seas is a map included in RTB 1.045. In this map, you are able to move around in any dimmension; you are able to float up and down easily, along with normal directional movement. If you cease movement, you will slowly sink or float until you stabilize.
Due to the aspect of this, this is one of the least popular RTB maps because it is more difficult to build in the water.
Astro made this map in 2005.
This map doesn't work if you have Bac's Pack.
From personal experience from a member who's been playing Blockland since 2004, I have only seen 4 entire saves on this map!
If you sink or float long enough, you'll encounter a brief period where you aren't in water.