You have a shOP manual? Should give you ossible reasons... 2 stroke or 4? If 2 stroke oil injected or premix? Pumpingn water good even at speed? Sometimes they can still pump, but not as much as they need past idle, the rubber actually gets stiffer actually from sitting than from use. If oil injected, id run through that system, then I'd check plugs for fouling and gap. Then I'd do the impeller... It's a 40 Min job max, and about a $50 part I'm guessing(mine are 45 for my '83 80hp merc) it's where I'd start...it's probably where the mechanic would too. Assuming it ran fine last year?