So I went ahead and dried up an entire Ocean Monument with only 20 sponges and a furnace. I planned to do this for a long time, and now here we are.
It's not far from land, so maybe I'll connect it with a glass tunnel. But gee em gee, this was a LOT of fun! Clearing out a Monument was something I've never done alone before, not to speak about sponging it dry. I can only recommend this activity.