Even if you're not much of a fish watcher you can still get a kick out of doing fun stuff on the ocean surface, such as parasailing, water skiing, wake boarding, jetskiing, or riding a banana boat.
For parasailing - dangling from a parachute at high altitude as it is pulled along by a speedboat - head to Scotty's, the Shangri-La's diveshop, who operate water sports activities under the Asian Action brand name. All resorts in Mactan outsource their parasailing to Scotty's, which has a boat for that purpose. Parasailing ranges from US $45 (150 meters of tow line) to $85 (350 meters of tow line).
For all other water sports activities, you essentially have a choice of Scotty's and Aquasports, at the White Sands resort. Note that jetskiing in Cebu is usually referred to as Waverunner, after the brand name. By the way, a banana boat is a boat which looks like a banana, not one which carries bananas.
There are alternatives even if you are not keen on diving but would still like to see the fish. While Mactan does not have any glass bottomed boats, you may be interested in a Sea Walker experience.