Most useful accessory

[CUunderH2O]

I was just reading the 'what have you regretted buying' thread, and realized that sometimes you buy something that you find really useful and worthwhile. I'm not talking basic equipment like BC/BP, regs, fins or mask.

The thing I bought that has been the most useful on all of my dives is my Princeton Tec "Tec 40" light. It's a tiny, but bright light that runs on 4 AA batteries. I bought it as a 2nd backup light, and keep it in my BC pocket. I attached a brass clip in case I need to clip it off if I'm taking photos. It has come in very handy for a lot of dives where it didn't seem to be necessary to bring a light, but having one made the dive better for me or my dive buddy. I can pass it off to my buddy if he/she forgot their light, or I can share it with my buddy on a daytime dive if we want to look under the wreck or inside it. It was well worth the $23 CDN I spent.

[Gary D.]

Drysuit!

Gary D.

[Mike Veitch]

Housings for the cameras

[Tigern]

I know it's cheesy but my aquatec noisemaker. Hooks in between the LP hose and Power inflator and makes a sound underwater like a duck quack. It's loud enough to be useful but not loud enough to be annoying. It works so much better then banging on the tank and the distinctive sound I have found attracts people's attention better then a metallic clang sound.
The noise from what I've observed doen't really disturb aquatic life much and since you only have to push in a button to get noise, there's not a lot of movement to scare of fish and such.

[ScubaJW]

For land, my marine life books. I am crazy about finding the names of lifes.

For water, sign language - to communicate with divers about things in the water and etc... Always use it on every dive!

[itemize]

Wrist mount for compass.

[redtide]

Clips of any kind: simple or retracting. I can't stand loose stuff! I also am glad for my save-a-dive kit, my dry box, and most of all, my safety sausage signalling device. Oh yeah, and a nice dive log book binder with all kinds of pockets to put business cards and random pieces of paper in that I always wind up collecting on a dive vacation. Simple and relatively inexpensive things that make the whole experience much nicer!

[nessum]

Longhose.
Neoprene mask strap
Sometimes I love my spring straps... sometimes I don't.
Ankle weights (not for my ankles, but as extra weight that can go just about anywhere... useful for an underweighted student).
Edit: Oh oh! And my pony bottle.

[offroaddiver]

Not including my general dive gear, overall I'd say my dive lights UK C8 and princeton tec backups. Everything else was either bought as a major piece of gear or has become a vital piece of gear (gear bag, dry boxes...etc.) Only thing that I've probably never been on a boat since I bought it was my Box-it cellphone case...That thing has saved my cell phones (3 so far) from seeing the depths of the brimy blue. I can talk on my phone (non flip phone) without worrying about getting water on it or dropping it.
Ok two things.

[blackman]

As an accessory....well....my little plank with wheels on wich I put all my equipment