For me, it depends on how I'm diving.
Recreational non-teaching, daytime diving- I may carry a P&S camera, a surface marker buoy, cutting device, compass, maybe a noise maker & my fold- up snorkel, if I'm not currently using it. For night time diving I will also have the standard primary, secondary & tank marker lights.
Recreational diving OW Teaching- I will have my teaching slates, a noise maker, cutting device, a compass (if I am teaching that portion) & I'm wearing my snorkel
. Since 99.99999999999........% of my teaching is in a quarry, I wear a drysuit & will carry a spare mask in the spare pocket, along with an underwater notebook & pencil in the leg pockets.
Technical diving- I carry my canister light, back- up lights, any needed reels, SMB- if needed for the dive, 3 cutting devices, both directional & non- directional line markers, underwater notebook & pencil. In my drysuit pocket, I will carry a spare mask.
I try to take along the tools I will need for that particular dive with some stuff for any contingencies & not much else. I don't like a lot of stuff hanging off me. It only creates more problems than it solves.