An Aquarius 21 probably isn't worth enough to justify a surveyor, but have at it if you cannot figure out the source of the leaks yourself. The forward hatch on an A21 (Peter Barrett design, BTW), is fiberglass. If it leaks, it either has to be where the hinge bolts go through the deck (needs recaulking), or the rubber seal tape needs to be replaced. If it is the side windows, they are made of a plastic pane with an aluminum frame. These can be removed and recaulked and put back in place. If it is the rub rail, then you have to remove the vinyl insert, which will expose the fasteners, then pull off the rub rail and recaulk it and put it back on again.
Other areas to look are any deck fittings, winches, lifeline stanchions, hinges, etc. where fasteners pass through the deck and they might need to be removed, recaulked with silicone and put back in place. Other places where water can get in might be from underwater through hulls, which might need to be pulled, recaulked (not silicone, but a polysulfide like Life Calk) and put back. Same with perhaps a lower gudgeon, or the cockpit drain (scuppers). The pin for the swing keel might leak and so that needs to be recaulked. You just have to rule out all the areas where water might gain access inside your boat from either rain, spray or the lake.