A friend of mine owns a Jetski Safari business in Australia, taking clients out on 3 hour trips on a fleet of 6 Yamaha FX140 Cruisers. All of them had a total engine breakdown at close to/approx 400 hours. Apparently the problem came from not enough oil being pumped through the motor, causing the pistons/cams to break. He even had a visting Yamaha technician sent from overseas to asses this apparent flaw, and was told to use them as spare parts as nothing could be done. As you would imagine, having 6 jetskis all fail, then be told nothing could be done was a little dis-heartening. He has sinced changed the models to Seadoo and is quite happy. The newest 2008 FX cruisers have a different engine design with much better oil supply according to the specifications.
All up, the FX 140 Cruiser series 2006 was a great machine..... just be carefull around 400 hours as we have seen here.