Strobes for many of the new digital cameras must fulfill requirements that are different than what has been required for film cameras.

Some of the digital cameras require a small initial preflash before the standard flash, allowing the camera to gain exposure information. The camera may not provide the best results without this preflash. The two flashes happen almost instantaneously. Most strobes for film cameras can not produce this rapid flash sequence.

Normal TTL strobes will exhaust their energy in the preflash and can not recycle fast enough to provide energy to flash for the photograph; or the strobe system must be capable of ignoring the preflash to produce a flash in sync with the main flash of the camera. Ignoring the preflash can cause camera exposure problems.

Digital prosumer (lunchbox camera)
Digital SLR?

If you are using the lunchbox models, you can get close-up photos quite successfully without a strobe, as your camera built-in flash may give enough light for the shot. But you should be real close.

The best way to go is to use an external strobe - as a slave flash. The strobe can be set off in 2 ways:
1. Some systems have a gadget (EV controller) that senses when your camera flash goes off and the sets off your strobe
2.Some systems connect from the strobe with an optical cable, you stick it with velcro to the front of the housing over the internal flash and the internal flash also senses via the cable when the flash goes off.
3. Very few housings have the option of a fibre optic cable going thru the housing to connect to your flash shoe.

If you are serious about underwater photography, the only way to go is with a strobe. This can set you back anything (dont hold me to it) between R7000.00 and much much much more.

The strobe system can be moved from one camera to the next, so if you upgrade, you can transfer your strobe to your next camera. I have even used mine on land.

For wide-angle shots you need a wide angle lense: you still have to get on top of your subject. Those lenses are expensive. You also need very good viz.

I have been shooting underwater form more than a year now, and I am at last starting to solve the strobe (flash) back scatter problem.

For me the important considerations were:
1. If anything goes wrong with my housing, I need/want a local agent
2. O'Rings: I need to be able to get o'rings fast, cheap and quick.
3. Ports: you need to be able to get the right one for your lens. I am able to borrow ports for lenses from my dealer.
4. Strobes: both Ikelite and Sea&sea have them easily available. Best to get the same strobe as housing - eg Ikelite strobe on 20d works with eTTL as far as I know.
5. O'ring grease - I want it easily available and cheap.

Prices on sea&sea is cheaper here than ordering from overseas. If you have flooding (and you dont!) you have an agent you can go back to. Overseas - forget.....Normally housings can be fixed if anything goes wrong.

I obviously prefer Sea%sea - one of the reasons I went with Nikon, but that does NOT mean Ikelite isnt as good. Ikelite housings are also cheaper than Sea&Sea.

For almost every lens you have a different port, and it costs.....

My favourite is 60mm, CT being suited to macro, my friends also like the 105mm macro. I have now dived once with 60mm +2xTC, great but slow to focus, but I can get very very close.

For wide-angle the lens of choice is 10.5mm (Nikkor) - dont know for canon.

Ikelite's ports are very affordable. The main guys bringing those in are here in CT.

To give you an idea -
my housing R14 000.00 (Nikon D70)
ports for macro R2500.00
Port and Extension for 2xTC with 60mm - R2500.00
Strobe - R8000.00 (strobe, arm and sync cord)

Ikelite is considerably cheaper.