Your concerns about extreme watertightness make me wonder if we have the whole story here. Are you planning to make this a boathouse that would be at lakeside, or do you perhaps live on a houseboat? In either case your "floor" would be water, and I could understand your desire to insulate your boat from the moisture. Some locations in bayou or swamp might be about as bad.

Unless you have some extrordinary situation such as this, you probably want a shelter that is NOT tightly sealed. You would want to have air movement to reduce the build-up of moisture from condensation.

Then there is the matter of cost.

Conventional styles of building have evolved to give durable structures at reasonable costs. The number of trade-offs and compromises possible in design and construction are too numerous to list here. Give us some more specific details about your location and storage needs and we might be able to suggest something more specific and useful.

Otherwise, it seems like you need a one car garage, or a simple plywood or OSB sided shed that is about 20 feet long.

PGJ