I've now gone back and fixed the broken multipolygon you mentioned. Thanks for bringing that to my attention.  I've also filled in the thin forest landcover gaps between my imported data and the existing data.
As for the waterways, I'm not sure how to better go about importing that data. I don't have the computational resources or the time to import whole watersheds at once. The disconnected fragments are not wrong IMO, just incomplete. OSM has a ton of incomplete data, it's inevitable.