The salesman may or may not be an underwater photographer, so I'd look at that first. Second, he may or may not have your best interests at heart - he may make more by selling one or the other etc.
I'd ask questions here and check out dpreview.com and digideep.com to get started.
I don't think you can go too wrong with almost any of the choices out there at the moment. In order to narrow it down I would decide which features, size and budget suit you and then short list. I think the A620 looks like a great little camera - I haven't had a chance to see one in action yet, but the A series before it produce some nice results. Remember, ultimately it's the artist not the tool - though the right tool for the artist can make life easier!!
For your bells and whistles...I would start with your camera and housing, a big card and an extra set of batteries. The second thing I would add would be a strobe, tray and arm. But I would get to know my camera first. Adding a strobe doesn't just instantly mean better photos - there's a pretty steep learning curve in using a strobe. If you have the cash and are happy to take a tougher road (you have to think about the camera and the strobe instead of just the camera) then go for it, I say. Sometimes I find learning everything at once is actually faster in the long run YMMV.
I'd add more lenses later - once I was comfortable with shooting underwater and once I had a better idea of what type of photography appealed to me the most.
HTH
