More @ Ken,
RedOxx bags have very high water resistance. So much so, that I don’t worry about them in heavy rain situations. My husband had a bag in heavy Florida rain. When he got home, he feared ruined work papers. Opened his bag and was greatly relieved to see so little wetness. His important work papers were protected. Dry, except for a minor edge near the zipper. I, also, had a bag in rain.
Same results. No water in bag.
The PUP bag is my daily go-to bag. It would make a good underseat bag and can hold 1-2 days of clothing in packing cubes, along with flight comforts. But, I recommend adding the comfort shoulder strap for long hauls around airports.
My husband uses the Gator bag for underseat airplane travel. It holds his papers and flight comforts. (No Clothing.) Stands up under a seat. Still has leg room.