Halcyon DAM
here or DSS DSMB
here for normal shooting, get a BAMF one like a Carter 90 or 120
here for bigger seas. The problem people have when they get big ones is they try and shoot it from a safety stop, and a 6 footer isn't going to fully inflate. You've gotta shoot it deeper, most of us will shoot a bag from the 21m stop or so anyway. It's full on the surface and easier to see, but keep in mind the bigger the bag, the larger it is un the water. Shoot the shorty, and if you surface and realize the boat isn't where you think it is, you can inflate the big one on the surface and don't have to worry about getting it full from shallower depth. DON'T GET AN EFFING LIFT BAG, YOU'RE NOT LIFTING ANYTHING, THE POINT IS TO BE SEEN!!!!! (couple general links lower re:surface marking at the bottom)
If you get one with an open bottom, make sure it has baffles. The fully open ones suck and deflate. Make sure it has an OPV. LPI's are easiest to inflate (although I've heard with cold lips it sucks, but I've never had a problem, and if you're on a rebreather, you'll be using the LPI plumbed off a bailout), no screwing around with an extra second stage, cleaner to deploy, don't have any buoyancy niggles. The ones with radar reflective material and SOLAS tape are good to have, they light up pretty good under NVG's and the Coasties radar.
Check the video below to see what a good deployment should be like.
Proper DSMB Deployment
Also, keep it wrapped and in a pocket or on your BCD. Set it up like this if you're not going to keep it rigged:
DSMB Spool Rigging
Spools are better than reels unless you're shooting it deep. Less likely to jam or birdsnest.
Couple other useful links:
"Best signalling equipment from the searchers point of view"
British study on surfaced diver visibility
To touch on the first link above, if I'm in the ocean and not diving with a can light, I've got my "come get me" kit on my harness in my can light position. It's got a PLB, dye pack, whistle, glowstick, and a granola bar in a Light Monkey canister with a blank cap. If I've got my can light, the kit is bungeed to my buttplate like a sidemount can light. If I were diving something like the Doria or some place with some wicked current, I'd probably pack a Halcyon Lift Raft as well so that if I were stuck, I could get myself out of the water.