Um... If a car falls off Sun Wheel, you're falling close to 40 meters in an enclosed steel box, do you really think you're gonna have to worry about getting out in a few feet of water?
Seriously, though - Sun Wheel at DCA is a big tubular steel beast of a machine, there are redundancies upon redundancies designed into the closures of it's gondolas, and it's inspected (both X-ray and nondestructive particle testing of welds) more often than any bridge in the U.S. I'd put my money on a catastrophic failure of that horrible parking structure before any ride at DCA.
(I'm not kidding about being more afraid of the parking structure than any of the rides, either).