I shoot with a 20D as well. My thoughts:
(1) a 2 strobe setup
(2) choice of lens for general and close up shooting (thinking 50mm 1.8 prime or 10-20mm Sigma or the 18-55mm)
(3) any other useful info I should consider for shooting underwater.
1/ Inon Z240s. Best strobes in the market for price/value.
2/ The typical 18-55 standard zoom is a good "all purpose" lens, but as far as I am concerned, it falls in a netherland - your main underwater shot types are fish portraits, wide angle and closeup, and rarely will a single lens suffice for all 3.
I have a 17-70 that I bought intending to use as my "scouting" lens, but it rarely goes on the camera. I find I get better results when I focus on one type of photography and spend time on it, rather than swimming around trying to shoot whatever I can spot "General" shooting is probably the best way to get mediocre results, IMO. Better to get specialized lenses.
IMO, a good 2-lens starter kit would be (1) the EF-S 60mm macro or the Sigma 50mm macro (both of which go to 1:1, I think - that is a lot better than the 50/1.8 for close-up work) and (2) either the 10-20 Sigma or the Tokina 10-17FE.
The 50mm macro works well for macro + fish portraits. I use the 100mm macro as i like the greater working distance it gives me - for macros only, it is probably a better lens. However, the 50mm is more versatile.
For WA, I own both the 10-20 and the 10-17FE, and the fisheye goes on the camera every time I need WA. But the 10-20 is a better lens if you want to use the same setup for land photography as well.
3/ Martin Edge's book on underwater photography, and Jim Church's essential guide to composition. Excellent books. And dive a lot. And start stocking up on 20D bodies
Vandit