You can get those togglers in stainless, I was surprised to see them in West Marine! However my choice would be to use a RivNut. A Rivnut works like a pop rivet. It has a head on it and internal threads. You use a threaded mandrel with an inexpensive tool that you tighten up to expand the back end of the rivnut. Then you unthread the mandrel and put your bolt it. Rivnuts work very well in fiberglass. If it ever gets damaged you use the same size drill to drill through it, and just install a new one. McMaster Carr among others sell them.
For lighter loads than a cleat I also recommend SwellNuts. Similar to a RivNut but made from rubber with a brass insert. They seal completely and won't crack the fiberglass, tolerate lots of vibration, and can be replaced at any time. Great for fastening interior hardware to fiberglass liners inside the boat, especially if foam filled.
Gary H. Lucas