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Ginny,
Whalers are famous for having no wood to rot, and thus no wood backing support to mount cleats. There is also no access to the underside to mount backing plates. Anywhere you drill, you are likely to hit a thin skin of fiberglass and then just foam. You can try some type of anchor like used for drywall, but in my opinion, those will tear out with any real force. If you have the stainless steel side rails, you can buy cleats that will mount to the tubing at West Marine. The side rails should be strong enough to hold the boat in position at rest with not too many people moving around on it, but if you are looking for something to hold the boat in a absolute solid position while trying to loan and unload it, you may bend the rails in the long term. In other words, your idea is much easier said then done. How about just getting some oversized fenders to protect the boat from the wall that will not , "sneak out", due to the current and using the standard whaler bow and stern eyes.

Jim G.