I have bought the cover for my seadoo when I bought the machine in 2005. It fits snugly and with the clips holds well. I have driven it on the highway at 120km/h+ for 2 hours + and have had no issues. I never thought of it flying off.. and luckily for me it never has. Never had any scuffing either. The vents seem to bloat it out but it works fine.
Just as an aside, I DID however, have rocks fly up from other cars and the cover saved me a few chips from that.