Here's a stand I made for a flag, but it will work for an umbrella as well. It's a short Lopro & pipe I took off an NRS side rail rack that I took apart. It already had holes in place. It can be moved to wherever you want on your raft, and the angle can be easily adjusted. Drill a hole through the umbrella pole & you're good to go. I added a short piece of PVC for strength with my flag pole.
"We are all visitors to this time, this place. We are just passing through. Our purpose here is to observe, to learn, to grow, to love....and then we return home."
Australian Aboriginal Proverb