Pretty straightforward, really. I was on my raft when I hovered my mouse over the waterfall and it said "Exit," so I tried it. At first I thought it was a clever wormhole idea but of course I was just walking out of bounds. I was easily able to jump back on my raft too. Hope this helps, I've always loved this game.
Really not sure what could have triggered it, but I was able to get out of it easily at least.