It sounds like they don't want the job. If it was actually a plugged main jet they would have had to pull the carb to check it and could have ceaned it in 5 seconds if they had the carbs apart.
BTW: I have never once seen a seadoo with a plugged main jet.
It is probably your fuel system and carbs in need of a rebuild. Probably about $200 in parts if you do the work yourself.