For an NRS frame umbrella mount just buy a LoPro fitting ($18 ) and a foot or 18" of pipe ($4/ft) and a couple clips and you're set. Cheap and easy, mine looks like this:
I use a big ol 8' umbrella, usually will cover the rower and person in front. Seems like with anything smaller those guys have to use more than one to get much coverage.
I don't worry much about items on the floor myself unless they are super heavy. I suspend the Yeti up front and an old cooler for a dry box behind the rower's seat. Water jugs on the floor wedged inbetween a couple frame rails to keep them in place in front of the rower, in the back I put my folding table down first to protect the floor then everything on top of it. Most of that stuff is not heavy though. Couple of five gal buckets next to the rower and saddle bags. No issues, like someone else mentioned, I have a Vanguard too and that floor is so thick and bomber I don't think a pinch from underneath is even a consideration.