Build a long narrow "pod" with vee shape and mount it under the deck. Build it from pt plywood and seal around the edges where it meets the deck. Then make a hatch through the floor for access.


The pod could have drain holes just in case water gets in and items would be stored in waterproof bags used for kayaks. I use the bags for kayaking and they are 100% dry, even if dunked. Cheap ones can be found at Walmart.


There is a company that makes pricy bolt on fiberglass sponsons similar to this in large scale for pontoon boats and claims higher speeds with better fuel economy.


I've got a 20' toon so know what you are looking at under the deck.