Speaking as a long time boater and rock climber, Never ever ever put anything on load bearing nylon webbing and ropes except mild liquid soap (dish soap etc.). I would highly discourage use of 303 on the straps. IF you are worried about sand, just rinse them off in the tub in warm water and hang dry, or even in a bucket outside. For longjevity, the best thing is to store them dry and out of the sun, away from all chemicals, this is what is recommended for cleaning runners/draws for climbing (basically the same material).
FYI according to Aire, 303 isn't even that good for your PVC boat, and should be used sparringly
"11. What should I use as a boat cleaner?
The best cleaner that you can use for your boat is mild soap and warm water. NRS offers an inflatable boat cleaner that works very well for grease and the tough dirt that won’t come off. If you are using 303 Protectant it should only be used a couple times a year. Using too much 303 will cause the plastic in the PVC to harden."