We just rented in St. Martin right at the beach off the boat for $40 a 1/2 hour and charged nothing for the extra rider in the back. They were brand new wave runners and in excelent shape, they also let you go out very far, really cool. This was on the 8/9 cruise, also rented 2 years ago at Orient Beach, but don't remember the cost, but there was no extra charge for an extra rider.
No wave runners at Megans Bay at St. Thomas.