I like a simple, modest-cost, easy to use tank light...having previously tried some popular products that didn't work out well over time.
I think this is the one (looks exactly like what I'm using) - the
Innovative 500 hour water activated flashing marker light - red. Small, lasts so long I don't concern myself with changing batteries or taking it off during the day, thus it's always there, comes on when I'm submerged, turns itself on and off...$12.93 and available with Prime shipping on Amazon.
Lycra socks to prevent chafing/blisters on high dive count trips.
Picked up an
XS Scuba X5 Accessory Hanger on a whim and like it! On a recent Bonaire trip, seemed like putting by dive boots on it hanging upside down and a bit open helped dry them out better overnight. It's not a BCD or reg. hanger, it's for fins, boots, gloves, mask...
If you want reef safe sunscreen, a tube of
Stream2Sea could be good. Trailboss123 spoke well of
EarShield, which I tried in Bonaire and seemed like a good product.
A Trilobite, or the DGX Sharp Cut with ceramic blade that's a lot like it, or other small, efficient and easy to carry cutting tool, would be good. A set of spring straps for fins. Maybe a folding pocket snorkel for people who don't like snorkels but might need one once in awhile? A water resistant note pad like Rite In The Rain for taking notes during dive briefings on dive boat decks where water's a hazard?