Debris
I was out on the Elk and Bohemia rivers on sat. There was a goodly amount of debris floating (literally flotsam and jetsam...), anyway, mostly logs, jugs, bottles etc. Nothing that would really endanger your boat I'd think. The relatively slow speed of sailboats and the teardrop hull will mean you almost never will see a 'direct hit,' and if you do, you'll be going slowly enough to result in a mostly glancing blow. There;'s always stuff in the water. Now, in a full-throttle speed boat or on skis or jetskis--thats another matter.