Merc 225 IO, is that a V-6 or V-8? I know Merc has a 220 horse 305, what year is yours? Just boring a motor 30 or 60 thousands over won't yeild a "good amount of horsepower" You'll get a small increase in power because you pick up 5-10 cubic inches, but not enough to justify just boring the motor alone. Typically you bore the cylinders to true up the walls. You get the power from raising the compression. If you do have a V-8, I'd concentrate on exhaust, intake, carb and maybe camming. Experment with props as well. If you end up using someone else to do your work, you'd be well advised to go to someone familiar with the needs and tricks of a marine engine, not a street car guy. As far as shops go, you might checkout offshoreonly.com. there are a bunch of east coast guys that hang out over there and might be able to recommend a shop.
Craig