SamsoniteDesigner Id Tags Pair

Q: Has anyone used this on American Airlines underseat item?

Yes, I recently used it on AA and it fit nicely. So glad I didn't have to fight for overhead bin. I zipped in my seat and out with this bag!

Q: Does the front pocket have any useful organizing slots or pockets or is it plain inside?

Yes. There are useful organizing slots in the front pocket.

Q: Will either of the side pockets hold a water bottle?

There is only one side pocket. You can fit a bottle of water (not a water bottle) in the side pocket, but it won't zip closed.

Q: Fits under seat & possible to fit: dress shoes, 2 pants, 3 dress shirts, 3 socks, 5 men's underwear, laptop, 1 shorts, 2 t-shirts?

In my humble opinion, all your clothing will fit. I used the under seat spinner as a second luggage. (Just in case my main large suitcase was lost) I packed more for a 5 day cruise. Enclosed was 2 dresses, 2 leggings with blouses, 2 shorts, shoes, bathing suits, makeup bag, wigs, curling irons and toiletries. It was full and tight but it fit. In your case I would hang my slacks and shirts as soon as you can. I hope I’ve helped with your question. Enjoy