When they ruled out mechanical did they tell you what each cylinder compression tested at? It could be electrical i.e. losing spark do a search online for a cruz tools spark tester or go to http://www.pitposse.com/gotspigte.html I have one of these and they come in handy for checking spark. Buy the spark tester it fits between the spark plug cap and the spark plug. Run the boat and when it dies install it and check for spark at each cylinder. If your pwc is the type that you have to remove the tip the spark tester has the same type of tip that can be removed. Before you go to the lake install it and start your boat so you can see how it works (should have a flashing orange light). It lights up pretty good. Also when you say it won't start are you saying it won't crank over, or it cranks over but doesn't start?