I read a magazine test of a jetski (I'm pretty sure it was in a bike mag) where they claimed to have whizzed off across the Solent at 50mph. Isle of Wight and back in a few minutes.
They may have been making it up and they certainly didn't go from Bournmouth. As Zak says, the sea can be very dangerous, I wouldn't do it without a well worked out backup plan. A tow rope from another jetski is not, IMO, such a plan.