Yamaha or Seadoo would be my suggestions. Seadoo is now using the Mercury powerplants in most of there boats. Yamaha has some great models also. Both have products in the marine business already. I would suggest staying with companies that have been in the marine business and have had great success in it.
When looking at used jetboats be sure to ask many of ?s about how it was taken care of. Biggest one being how often the motors were flushed out and so forth? Was it winterized properly every season and who did the service work?
Feel free to PM me with any questions or concerns you may have.