I think you can do nice work with the Olys with available light or their onboard flash. I'd definitely spend some time getting to know the camera before buying strobes or other add-ons like lenses.
I came to the 5050 from a year and a half or so of freediving and taking pictures with a Nikonos. I use the Nik mostly at one of the two extremes -- either with a 15mm wide-angle lens, or with extension tubes to shoot macro of creatures like nudibranchs. Since the Nikonos is a rangefinder and not an SLR, though, it's hard to take standard 'fish portraits' from a couple of feet away. One of the main things I like about the Oly is the capability to shoot such fish portraits.
As for macro, you can get quite close with the onboard lens system. I did buy an add-on macro lens, but haven't used it much yet. There are some sample pictures at the bottom of this retailer's web page that show the effect of various lenses:
http://www.uwdigitalcamera.com/English/eM67lens.htm
As for batteries, rechargeables are the only way to go. I use 2200-mAH Maha Powerex AA's for the Oly and strobes. To conserve battery life during a diving session, I leave the camera turned on but I turn off the LCD screen, which is the main power-munching function.
The other comment I'd make about the 5050 is that it has so many different functions that it takes a while to get to know it. I'd definitely recommend using it as much as possible topside so that you aren't spending all of your underwater time on the trial-and-error part of the learning curve. I've seen people take great pictures with a 5050 (much more awesome than mine), so I'm sure you'll enjoy it.