I just bought one from Dick's Sporting Goods that is really a beach umbrella that doubles as a shelter (on sale for $69). It has some flaps on the sides that can be staked to the ground (as can the rest of it) so that it forms a neat little lean-to like shelter or shade. Because the rim of the umbrella has d-rings around the perimeter, I figure I can tie it to the raft, making it less suseptible to wind. It also has panels on each side to let some air into it (when it's a shelter in particular) that zip open and closed. It's also vented at the top like a golf umbrella, also helping resist damage from winds.
The down side that I can see is that the pole is a narrow diameter, and it is pointed on one end (so it can be driven into the beach!). I plan to turn the bottom section around (it telescopes in and out anyway, so no big deal) so the point is inside the outer tube and then maybe put a sleeve around it so it stays in my umbrella stand better. it is also heavy, because the poles are sttel rather than aluminum, but other than those two downsides, I can't see anything wrong with it.
I'll be testing it on Ruby/Horesthief in a couple of weeks! Until then, all I can say is that it's a good idea if nothing else. And worst case is I keep it for soccer games this fall... LOL
It's not listed on their Web site that I can see, but it looks something like this (girl not included LOL):