Neoprene - 1) TULIP brand paint pen about $4 at Michaels Craft Store; use hot pink (yes - even you guys - what man's gonna steal something marked in hot pink?), lime green or bright orange; I mark my 1st initial; apply the paint in thin line & let it dry a full 24 hrs. 2) Old fashioned round hole punch or 2 - 3 punches in a pattern.
Hard/semihard/flexible plastic (fins, snork, etc.) - Definiely agree w the Silver metallic Sharpie huggers. Mine always gets "passed around" the boat, so I bring 2, just assuming I'll never see the first one again. I don't mind. My donation to the boat. : )
Cam/housing - Preprinted address return label, covered w slightly larger rectangle of clear postal tape. Careful where you put it. Good for any photographic equip., whether used u/w or not.