Phil, it seems like you and I are in violent agreement. We're both saying that the E-620 is a great camera, and there are great underwater accessories for it. We disagree on one thing: I don't think a full underwater setup with an Olympus dSLR is that much cheaper than a setup with a Nikon or Canon dSLR. I think that when you price out a full system with housing, lenses, ports, two strobes, and strobe arms, the Olympus system will cost around $5,000 to $6,000 (although you can certainly start out for $3500 or less), and you can get a Canon or Nikon system for about that price, maybe 10% more. Ikelite housings are more expensive than Olytmpus housings, but you can get cheaper domes, and in the end it starts to even out.
Regardless of the exact price differences, I don't think anyone should buy their first dSLR system based on a 10% difference in price. Look at Phil's story of starting with an E-300 and re-using lenses, ports and gears in an E-520, E-620 and E-3. When you buy your first interchangeable lens camera, you are committing to a camera system for life, and saving a few hundred dollars over the rest of your photographic life might be a mistake.
If you want to get an Olympus system because the Olympus 50mm macro lens is the best macro lens in the world, go for it.
If you want an Olympus system because Olympus has always made the smallest SLRs and size is important to you, go for it.
If you want an Olympus system because you like their software (I'm a big fan of the "properly expose shadows but blow out highlights" and "properly expose highlights but lose the shadows" modes which I haven't seen on Canon or Nikon), go for it.
If you want to get an Olympus system just because it's cheaper than others, I would ask you to consider the total cost of ownership. If you ever decide to switch to Canikon and have to sell your Olympus gear, you will have lost much more than the 10% cost difference.