I've got the A520, not the A620, but they're similar. I've been to Palau; a lot of the shooting you're going to do is going to be wide angle with a lot of ambient light. If you can get really close to your target (6' or less), strobes will be great. But, if you're at Blue Corner or Pelilui Express, you're going to use a reef hook and strap in. You probably won't get that close to your subjects, and you'll be in better shape using white balance on dives like that. You'd need a really long strobe arm to avoid backscatter. During the day, you'll probably get nice stuff without a strobe anyhow. The Palau photos in my gallery didn't use white balance and used an S50, with a big heaping piece of post processing. For those, I'm not wishing I had had strobes, I'm wishing I had taken the time to get good with white balance (which I did right before I flooded my S50 housing).
With the A520, there are shots in my So. Cal album that used manual white balance as deep as 70 feet, and I got pretty good color between the manual white balance and post processing (the Ruby-E wreck shots). I would think the A620 would be better than my A520, its got the digic2 processor. If you're going to night dive with the camera, of course you'll need a strobe (sorry, no recommendations for the Canon, I use a C5050 with a Nikonos when I'm using a strobe, and use my A520 strobeless). During the day, you may be able to get by with the internal strobe for macro, and white balance for everything else. A lot of the dives are wall drift dives, so strobes for shots of coral and barrel sponges would probably benefit. You'll also have a few cave dives (Blue Holes, Chandelier Cave, maybe Saies Tunnel) - not much ambient light, you'll need strobes to light the foreground and then set the appropriate settings to get the backdrop. If you can afford the strobes (like the Fantasea setup suggested), get 'em, you won't regret it. Just keep in mind that it may not always be appropriate to have them firing.
