Selecting an operator to go with is difficult as there is really only one criterion visible to the visitor to base their choice on and that is price.
As a general guide, one round of parasailing or a 15-minute ride on a banana boat will cost 100,000 rupiah. A 15-minute ride on a jet ski will cost 200,000 rupiah. Often operators will have two price lists — one for locals and one for foreign guests. The first step is to ask nicely for the local price list and then commence negotiation as even the local price list is an ambit claim.
You will typically see families opting for banana boat rides and younger people opting for jet skis, but other more bizarre options are on offer such as being towed on an inflatable raft that once at speed takes off out of the water with the occupant hanging on for dear life.
While few problems arise, there have been fatalities in the past, particularly with the parasailing, so do take care and try to select an operator that seems to know what they're doing.