My first guess would be the wear ring, what kind of shape is it in?
Sorry should have mentioned I had the wear ring replaced and it did not help.
I know 2005 was the first year for the 18ft Challenger, the first problem I encounter was porposing even in calm weather. The dealer installed trim tabs that seemed to eliminate this problem.
Not sure if 2005 being the first year Seadoo had not worked out all the bugs?