Gutters and awnings to keep the rain and snow off the porch is more hopeful than any waterproofing attempts. In any case, with properly installed t&g, the amount of water draining through is minuscule. Creating a floor that is as waterproof as a roof is possible, but at more than double the cost; you build a flat roof, with a membrane or ice&water shield material, over exterior plywood, then place sleepers (spacers) to which the floating t&g floor is attached. The sleepers have to be applied in such a way that they don't perturb or (especially) puncture the membrane.
The walking floor may be T&G fixed in place permanently, or can be made of floating sections of deck boards (with spaces between) and fitted between columns or rail posts in the case of an open deck.
This degree of waterproofing requires pre planning to account for the extra 2.25" of thickness, and will throw your porch stairs (and possibly the door to the house) off kilter.
You have to use sleepers so the water can drain and air can circulate. Wood flooring will not tolerate being laid directly on another solid backing out of doors.
The best flooring to use IMO would be mahogany (even if painted) because it has excellent characteristics for outdoor use. It is very rot-resistant, and is the most dimensionally-stable wood. It's very dense and hard and takes paint exceptionally well. My advice is to back-paint all of the flooring ahead of time, which is important in keeping the material from swelling and ensure tight joints.
If you want a really smooth surface, the flooring can be sanded after installation just as an interior floor, and then painted. Oil-based porch & deck paint works really well on mahogany.
About your crawlspace: Pay attention to the grade outside the perimeter of the porch. Over the years, soil will build up around a house, and today the grade could be quite a bit higher outside than under the porch, which condition invites water to pour in during rains. You could raise the crawlspace, or re-grade outside to encourage water to flow away. Plastic sheeting laid on the ground undr the crawlspace and secured down with a few inches of pea stone makes for a clean and moisture-controlled area.
Casey