Marine Grade Plywood?
Again if you can''t afford the marine grade plywood, you can''t afford to build this boat. If you assume that in your current configuration the boat will have 24 sheets of 1/4" plywood, the cost difference between using marine plywood and exterior plywood of equal species and thickness would be roughly $200 for the whole boat. Frankly, you cannot afford NOT to use marine plywood.
The 18 gauge steel idea makes absolutely no sense at all. 18 gauge steel would add cost, would add a lot of weight, would be next to imposible to bond without creating a whole bunch of colateral issues and would not really help with impact at all. I think from an impact resitance, durability, construction time, and cost standpoint that you would be far and away ahead of the game looking at a vinylester- directional fiberglass or even a kevlar sheathing solution.