I ran the horns with an 18' boat, and also the left line at Lava. It was not high water at Lava, but at Horn it was. Horn was my favorite rapid!
I still think it is too small. You will flip, probably multiple times. That is a pain in the ass to the rest of the folks on the trip, dealing with your carnage, reflipping, etc, etc. The water is cold and swimming has real risks.
I think you'd be like a pin ball in a pin ball machine. That may sound fun, and would probably be on a day trip, but not for 18-21 days if you become a burden on the rest of the trip.