How can I fix these gaps.

Those gaps are necessary so the snow doesn't melt and puddle and rot the wood faster.

Can you staple clear plastic directly underneath?

We had a similar problem and we installed a French drain.

Put in a Wooden or Plastic Beading and seal it with Bathroom sealant.

You can get inexpensive, corrugated fiberglass roofing panels and install underneath the decking to drain the water to the outer end of the deck. Be sure to have a slight angle so water drains away from the house. Doesn't look bad and as a property inspector, I've seen many decks with this done.

The porch is made with spaces between the boards so the water will flow between the boards. Standing water on the boards will make them rot quicker. Make sure the ground below the porch is sloped away from the foundation of your house.

If you were close to me, I'd do it for you. But, since that's not the case..........basically you'd nail (galvanized) it up to the joists every 6-8 inches. Overlap 2-3 waves and continue. Also, where ever you buy it, they can give you some guidance too. Farm stores, HD, Lowes, etc.

I don't have any pics, sorry.

Well, here's a pic , galvanized steel might be cheaper. This is from Home Depot site.

With all the things that are important to take care of a house, this is really pretty minor. The spaces certainly are not hurting anything. The most important part of this whole thing that is worrisome, is to make sure the ground under your deck slopes away from the foundation of your house for water runoff.

LOL.......depending on the measurements, you may not even have to cut any pieces. No these are already corrugated, no bending required.

Q

Go to this web site:

https://www.wikihow.com/Build-a-French-Drain

This requires effort but the water under your deck will drain away.

YES