Your owners manual should tell you how to winterize but Here are some threads and info on how to winterize your Yamaha (will apply to other jet boats too):
- Winterization Checklist
- Winterization Video:
- Winterizing a 2 cycle Yamaha
- Rotax Engine Winterization
- Another winterization video (there are disputes about if you need to fog your engine, if putting anti freeze in your engine does anything etc,)
- Another winterization Thread
- Another recent (2016) discussion on winterization
- Ronnie's 1.8L winterization on you tube https://jetboaters.net/threads/fogging-yamaha-engines-how-to-video.11477/