At the water's edge where the waterfall animation is, using a log raft I could exit onto the out-of-bounds clouds. I walked around half the map and was able to build a new log raft at a different waterfall and hop back in bounds. I tried to build a fire out of bounds which did not work. I was able to be hit and take damage from a Thunderbird while out of bounds. After traveling about a screen and a half or two screens away from in-bounds surfaces, the floor cloud textures disappeared and I could walk on blackness but the animated poofs that appear above the clouds were still showing up. All of this is recorded if you would like to see a clip.
Build log raft, swim to waterfall animation near out-of-bounds clouds, exit when "Exit" appears above clouds, walk around, hop back on log raft. Did not test with other boats.