I want to take some photos with a digital camera while scuba diving. I have heard that strobes are better to use. How do they work? Can you just buy a separate strobe and connect it to any underwater digital camera? Thanks.

A stobe is either slaved directly to a cameras flash port ( if it has one) by a synch cord or it’s slaved via the cameras actual flash itself . In the former’s case, the actual on board camera flash is disabled. In the latter’s case, the onboard flash isn’t disabled but an opaque shield is placed over the flash, allowing the flash discharge out sideways, triggering the stobe’s own discharge via a small sensor.
If what you have is a normal point and shoot camera and a decent housing that has a strobe tray mount or capable of accepting some mouting hardware, you can go this route. IkeLite for example, makes a housing for many makes and model cameras, that allow the use of an aftermarket strobe. No wires, fibre optics and no connectors between the strobe and camera inside the housing. It simply triggers off the actual light flash of the onboard camera flash and an included flash shield stops the onboard illumination from heading to your subject but allows the offset strobe’s light to hit the subject instead. Voila…no backscatter in your image. The strobe’s light hits the subject at an angle relative to the camera lens, illuminating the subject but the light reflected off particles in the water outbound to the subject doesn’t head right back into the camera lens before the subject’s light, like would happen using an onboard flash. That’s how a strobe eliminates backscatter in a nutshell and why you see strobes attached to the camera housings by flexible arms. It keeps that strobe’s flash of light away from the camera lens and at an angle to it.
Here’s a few examples of strobe units.
this one in particular is probably going to do the trick for you http://www.ikelite.com/af35/autoflash.html but check to make sure that your camera housing is supported for the mounts. I use the dual version of this strobe on this housing for my Canon 720 IS http://www.ikelite.com/web_two/can_a710.html .There’s no mount compatibility issues because the housing and strobe are Ike Lite.

If your camera is what’s called an amphibious camera, like a Bonica or Sea&Sea and if it’s relatively new, it may be set up to accept an optional strobe from from whatever camera manufacturer ( Bonica or Sea&Sea) you own. Older models ( 8 years ago or more) required you to jump through a lot of hoops and do some modifications to set up strobe funtionality for them.
You need to understand that it’s difficult to point you totally in the right direction here without knowing what camera you have, what your budget is and if it’s worth it to you to go the extra expense if you’re only taking a few fun shots or doing wide angle wrecks and macro life.
You don’t need to own an F1 race car to get to work a block away unless it’s imperitive you’re the fastest one on the street is what I’m saying. 🙂