Maybe if you are an extreme 'minimalist freediver' you might go in the water with only a swimming costume and mask / goggles (see an extremely impressive take on this in the video below!)
- (Johnson's Baby) Shampoo -A cheap and little known way to keep your mask from fogging up. Apply inside the lens before getting in the water, and rinse out before putting on, to guarantee a clear view all session! This has saved the day for me or my students more than once after masks that should have been good after multiple treatments with the flame of a lighter, still fogged up.
- Fin Retainers - Cheap and small these can mean that if you forget your neoprene socks you can still use your fins and dive. If someone else with smaller feet needs to borrow your fins they can still be used. I use them all the time as it gives a more secure fin fit and transfers more of the power each kick to the blade (See: https://massdiving.com/products/full_foot_fin_retainers)
- Emergency Whistle - Small and discrete I have these attached to all the buoys I use for teaching. Perfect to attract attention or give a warning in an emergency situation (See: https://www.ndiver-rescue.com/peeless-whistle)
- Mask strap - Can save the dive session if you mask strap breaks, and it will take up almost no room in you mask box! A must have.
- Lanyard - Even if not diving deep or diving off the line I keep this in my dive bag. If used as a backup lanyard on a line training session it gives an option to secure bi-fins or monofin as they can be threaded through and kept secure when not in use (diving no fins for example). It also provides a spare carabiner that is always useful to have.
Have I missed anything? Let me know in the comments section if you have any more to add.