I had a similar experience running into a dead-end channel between plains and swamp, I tried turning the boat around, but some fuling swam out to my boat and started pounding on it. It soon became clear that the boat wasn't going to last much longer, so I decided to abandon ship and see if I could make it to the lesser of the two evil places, the swamp.

I was able to make it to the swamp, but started getting converged on by swamp creatures, even while I was still in the water. I made it to large horizontal branch that was partially in the water, so I climbed up on it. When I got to the highest point, I looked down and saw the fuling and swamp creatures doing battle.

I don't remember how, but the creatures were so distracted they ended up losing interest in me, and just stood there until morning. When it looked like the coast was clear enough, I climbed down, swam over to the remains of my boat and cargo in the water, re-built the boat and sailed off.