I take two cameras with me. Both are older model Canon 20D's. I have shot so many photos that I never take it out of manual mode at this point.
One 20D has a Tamron 17-50 2.8 on it that has never come off in 10 years. This is the non VC version. I am very good at being steady for slow shutter speeds. Tamron does make a VC version of this lens which is more expensive, but I have only used mine twice. I found it very loud and slow to focus. Canon makes a great lens in this range with IS and at 2.8 as well, but it is bigger and heavier. And more expensive. Overall, I have found the small Tamron non VC version to be a fantastic workhorse. It gives better natural tones in warmer colors like red and yellows (skin tones, etc.) than Canon which is much better in the cooler temp color like blue and green.
The second 20D, I rotate lens on it. In Sydney, I went to Bondi Beach and shot some photos there from the beach with my 100-400 f4. You don't need a bigger AV range than that for beach shots since there is so much sunlight anyway. I have a Canon 85 1.4 that is simply awesome for portraits and studio modeling. The 1.8 version is great too at a fraction of the price. I sometimes use fisheyes lens, wide angle lens (Sigma is better here), or the Canon 70-300 IS II USM lens which is a fantastic telephoto on par with their "L" series but is much better for walking around than my bulky 100-400 lens.
As far as actually going in the water and getting wet, I wouldn't take my gear in water over my waist. If I had to do shots like that, I would simply go to a camera shop and rent a waterproof setup for about $100 per day.
When I was in Port Douglas, we did a snorkel and dive trip over the Great Barrier Reef and they had decent little cameras for rent for $35 the entire day. I was amazed at the quality of photos they delivered at 10 feet underwater for such a small camera.
But if you
