Macro is "easier" to learn and you can shoot it virtually anywhere.
It sounds like to get started, your zoom lens, again if there is a port for it, might be a great place to start. I've taken my Sigma 18-200 for several recon dives and while I never thought I'd put this thing underwater it's proved a good choice for those dives where I am not sure what I will find, what the conditions will be like and/or when I can't make up my mind. The downside to these zooms is that they don't focus close enough for some things!
You 20D underwater will not feel the same as it does on land
In all honesty, I'd go for one of the other digitals until you decide what you really like to do...unless you are happy to a) keep throwing money at this and b) happy to be limited by your lens choice once underwater. I can't tell you how many times since moving to the DSLR that I've wished I was using my Oly 5050 so I could do the opposite thing (macro when I have wide angle on or wide angle when I have the 100 macro on)!
It sounds like to get started, your zoom lens, again if there is a port for it, might be a great place to start. I've taken my Sigma 18-200 for several recon dives and while I never thought I'd put this thing underwater it's proved a good choice for those dives where I am not sure what I will find, what the conditions will be like and/or when I can't make up my mind. The downside to these zooms is that they don't focus close enough for some things!
You 20D underwater will not feel the same as it does on land
In all honesty, I'd go for one of the other digitals until you decide what you really like to do...unless you are happy to a) keep throwing money at this and b) happy to be limited by your lens choice once underwater. I can't tell you how many times since moving to the DSLR that I've wished I was using my Oly 5050 so I could do the opposite thing (macro when I have wide angle on or wide angle when I have the 100 macro on)!