Deck build in progress.
Five feet wide floating deck (no pun intended) around a 24 foot pool and on the side towards the house will be a connected ~20 x 16 section jutting out. All pressure treated pine on decking blocks.

Link to unedited images. (let me know if link doesn't work) Just over a week working on it so far, just me with no help so don't expect too much!
https://postimg.cc/gallery/2qef7qpf2/

Anyway...
I planned on leaving the space under the deck open for a couple reasons, it ranges about two to three feet high... I thought I would just spray / kill the grass and cover with landscape fabric then some cheap mulch. Cheap and easy right? Now I see on the interwebs that mulch is not recommended because it would get and stay wet and be stinky and the pros suggest rock / gravel and no landscape fabric.

The pool is only used once or twice a week with the occasional large gathering.

What do you guys think, I can't weed wack it under there and don't just want to leave dirt...

We have coarse white gravel under ours. At most, I just spray the edges.

Bodies of your enemies or pool floats.

As you have already found gravel usually #57 or river stones or something in between. No mulch for sure.

I had river stone under my pool deck, and all around the border of the pool. The previous owner actually did it. It looked good, but now that I had the pool removed I’m repurposing it for landscaping around the house.

I had a raised deck in Massachusetts, and I left it as raw dirt underneath; no gravel. I did screen in the sides with lattice work, to keep kids, dogs, etc. out, and I left one panel hinged so I could get under to retrieve stuff that fell through the cracks. I had no problems with smell or stuff growing as it was completely dark under there.

Looking at your pictures, it might be a good ideal to put the gravel in, and it will make spraying the roundup easier and will look better.