The fiberglass wouldn't be essential. It would just add rigidity and keep the dings down. I've seen boat docks supported with styrofoam blocks with no skin. The trick is to have a rigid frame on which to mount things. The foam would serve only as floatation with no structural stress placed on it.
Afterthought: Instead of fiberglass, you could have the blocks heatshrinked with that material they use to winterize boats. A strong polymer which would protect the foam somewhat from dents and dings as well as giving the pontoons a nice finished look. Of course you'd have to carve the foam into some nice swoopy-curvy shapes