I don't have a 510, but a 330...yet I've read a lot and can try to answer some of your questions.
1. Read the 'story' pages at
Four Thirds. In short as I understand it, they decided on a smaller sensor and an aspect ratio that permits the lenses to be smaller and lighter while focusing light 'perpendicular' to the sensor face. The reduced size supposedly also results in getting better equivalent depth-of-field for the same equivalent zoom ratings and f-stops compared to systems that just followed over from 35mm film, with less distortion out to the frame edges. Disadvantages are that the sensor is smaller so noise floor, color bit depth, etc might be more limited than in full-frame sensor systems. To me, at my level of photography (novice - never shot underwater with other than an old 4MP Canon before), the 4:3 standard lowers the price bar and makes it very attractive for me to try moving up to a dSLR without selling a kidney.
I don't know if some old lenses might still work with adaptor rings - perhaps solely for passing aperture data but certainly not for autofocus. And since non-4:3 lenses aren't "designed" for the 4:3 format, there's probably not just a conversion assumption (the 2x you see quoted a lot in reviews, e.g. a 50mm 4:3 macro lens is equivalent to a 100mm for the old 35mm film standard) but also some potential loss of the usable extremes for zooms and the like. I probably wouldn't try it for other than prime lenses.
2. I think the 4:3 system has a great array of lenses at quite reasonable prices compared to the big 2 (Canon and Nikon). You'll no doubt get all sorts of purists explaining to you that you never recover from not having a full-frame sensor, but reading at dpreview.com there seems to be a general feeling that the Olympus (Zuiko) kit lenses are probably the best kit lenses out there among any manufacturer, for their price. Introduction of the newest 4:3 lenses is also pushing down the price for the older yet still quite good semi-pro lenses (e.g. the 14-54 can be had for on the order of $300 used, which once listed over $600...). As far as shaky hands goes, I've got them too, but didn't wait for IS. The way I look at it underwater photography is all about light, so you have to have strobes anyway. IS might let you bump down an aperture stop or two compared to not having IS, but it won't replace having strobes, and needing to keep your exposure time down to the 1/80th of a second or less anyway to shoot fish that are moving. So I didn't see the cost advantage to going IS. But if I did want it, I'd agree with you: I'd rather get it in the body than the lens.
3. I don't have any insight into overseas pricing. Generally the 4:3 brands are Zuiko (Olympus) and Sigma. Sigma generally seem to be a bit cheaper, but I generally see the Zuiko lenses recommended over the Sigma for quality and performance over the quoted range (less distortion at the wide open end, bigger apertures over the whole range, less chromatic aberration, etc. for 'comparable' zoom ranges). Like I said in #2 above, there's also some product replacements going on that helps drop the price on a lot of the (still very good) Zuiko's like the 14-54 since the 12-60 has been released. If I was buying a new camera today I'd buy perhaps just the single-lens kit with the 14-42, and then search ebay for a 14-54 for a high-quality 'walking around' lens, and try and score one of either the newer (4 - 5.6 aperture) or older (3.5 - 4.5) 40-150mm lenses cheap, under $100, if I really wanted more of a zoom in the near-term. If you're really serious about zooming then skip the kit lens up to maybe the 50-200mm or so, but that'll run you about $550 or more. For underwater you really don't need more zoom than the 14-42 or 14-54 allow, but would add a macro or two and perhaps a wide-angle. I'd love to get the 7-14, but that's getting pretty pricey.
Oh, and before the end of January Olympus is doing rebates on a lot of the lenses too - check
Home - Olympus America Inc. for the listing. So I don't know that there's a lot of advantage to be had buying 'local' in Asia.
4. Any camera, any brand, is not going to like salt water, salt humidity, and sand getting into sensitive spots (between the focus ring and the lens body, etc). You're going to want to use a modicum of care regardless of what you choose; it's just the nature of the beast. That said, a lot of the nicer (mid-grade, not kit) 4:3 lenses do seem to be environmentally sealed, which doesn't mean waterproof but does mean if they do get splashed you can rinse them carefully with clean water to de-salt and should hopefully be alright. From my experience so far with the E330 I can say it seems to be a very solidly built little camera - no creaks or whatnot if you grip and try to 'twist' it a little. I've used it out in a drizzly-fog sort of day and haven't noted any ill effects as a result. As the top of the pro-sumer line, I'm sure the 510 is the same. While the cameras themselves aren't environmentally sealed like the new E-3, Oly I think is the first and best at dust removal from the sensor.
