Plywood. Marine grade is pointless if you gonna glass it or seal it.
For the record, very few people know the reason for "Marine" plywood.
Marine plywood has no voids (areas in each ply that have knots or imperfections have been filled with epoxy or plugged with more wood) and the glue is water proof (most exterior plywood's glue is water resistant, not proof).
Really the only time marine plywood needs to be used is when the plywood is going to be exposed to direct water contact (even if it is coated with resin)
You cannot completely seal wood with just resin, the reason is expansion and contraction, the resin will just crack (like varnish), to better seal the exposed areas of plywood, you should coat the parts with a laminating resin (no wax) and cover with fiberglass cloth (finishing is ok) and recoat with more resin until all of the voids and imperfections are filled, then go over with finishing resin (contains wax to cure the surface). Epoxies are the hot ticket for this because of the product selections available, you can literally choose the viscosity of the resin to be used...
Always use kiln dried lumber if you are going to apply resins, it will actually draw resin into the pores, wet "lumberyard wood" will not pull in nearly enough product to stick well or seal virtually at all....I like Birch for it's straighness and Fir for rigidity (especially over long spans like stringers)
School is out
GT
