ScubaBadger32
Contributor
how important is it to have a DSMB, and should i get one? what do i need to look out for when getting a DSMB and reel?
DSMB importance
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VWRdiver
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DSMBs are especially important when diving big water. For instance, when boat diving in current, it is quite possible to get swept away from the dive site, and the boat. DSMBs make you much more visible so you are more easily seen by a boat, even from quite a distance away. I would scale my DSMB for the type of environment. Ocean diving with high currents generally you want a bigger one than boat diving in a lake.
If you're in open water, you should probably have one (on a thumb reel)....and a whistle and a signal mirror. 
I never dive without one and I typically use a thumb reel/spool. Get one you can easily fill at depth. That will be determined by your preference. Some have mouthpieces/tubes, etc and some fill from the bottom with your reg set, etc. I like the open bottom fill type because I shoot a lot more buoys from depth than I fill on the surface. But the main thing is to have one, any type. There are many good brands and reels out there. They save lives.
For UK diving it is almost unheard of not to have a DSMB when OW diving. Most boats have it as a minimal requirement. It is a skill element of Sports Diver (the second BSAC qualification level).
Anywhere there is tidal water, a DSMB is a significant safety tool. It allows the boat to track you as you leave the wreck (whilst underwater), if you are doing stops or only a safety stop. It is also an excellent signalling device once you are back on the surface.
Generally Red is easier to see when you are on a boat (looking for divers), and Yellow when in a helicopter ( according to coastguard exercises).
Gareth
Anywhere there is tidal water, a DSMB is a significant safety tool. It allows the boat to track you as you leave the wreck (whilst underwater), if you are doing stops or only a safety stop. It is also an excellent signalling device once you are back on the surface.
Generally Red is easier to see when you are on a boat (looking for divers), and Yellow when in a helicopter ( according to coastguard exercises).
Gareth
Interesting. I've read plenty about people shooting DSMB's as a regular practice when surfacing and doing a deco stop, and have wondered how someone on the boat can differentiate between that and a DSMB that means "I'm in trouble". I suppose it might be determined by what kind of diving you're doing - unless you're in a situation where entrapment might be an issue, why would you shoot a bag to say you've got a problem when surfacing would be an option.
VWRdiver
Registered
Also, make sure you get one that has a relief valve if you plan to use it at depth so it doesn't pop on the way up. There are ones that are just called SMBs which are designed to be inflated only at the surface.
All signals really need to pre-agreed. But the standard simple signals are
Orange/Red DSMB = All Good
Yellow = Problem (needs investigation) {1}
Red + Yellow (2 SMB's) = Out of gas issue - Deploy drop cylinder/safety divers. {2}
{1} My yellow DSMB has laminated standard messages that can be torn off leaving the most appropriate.
{2} if you are using drop cylinders they need to be pre-rigged, and the boat needs to aware of the action to take. This is all part of the initial dive planning and briefing.
If we see a Yellow + Red we would put divers back in to determine the issue and offer support/gas.
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