I'll build a shelf extending out from the top of the header, inside the shed, above the big doors but below the transom hatch. Then I can secure a box fan in place and push air out one end thus pulling cooler/dryer air in the other end. This combined with keeping a fan running in the boat should keep the mildew at bay. What I like about this option, aside from the fact is was the most inexpensive way I could come up with, is that I can still open the transom hatches when I am working on the boat. I get the best of both worlds.
I think I may also need to lay some ground cloth out on the grass/dirt area between the plywood floor work areas on each end to act somewhat like a vapor barrier. I think that the moisture in the ground contributes to the humidity in the shed when the sun starts beating down.