Thanks, MG30D!
I use a G7-dedicated housing from Ikelite, along with a pair of underwater strobes. Not cheap, but strobes are a necessity for underwater photos past 3-4m (15-20ft) deep. They bring back the colours that are lost. Those two photos were taken around 15-25m (45-75ft).
I'm assuming you'll be snorkeling, not diving, based on your camera choices. If that's the case, I'd still recommend the G7 + housing. The camera takes great images and has a decently wide angle lens (a plus for underwater). The factory Canon housing works great as well.
Housing vs Dedicated Underwater Camera:
It can be nerve-wrecking to use a housing, but as long as you take proper precautions, it is safe. This is my normal checklist always performed the night before use:
1. Always keep the oil ring properly lubed (lube usually comes with the housing).
2. Check the O-ring in detail to look for signs of cracks before use.
3. Perform a test in the bath for leakages (without camera inside housing).
4. Pass the test? Open up the housing, place camera inside, and repeat steps 2-3.
5. Once done, leave it in a room temperature area with towel draped over the housing. Avoid direct air conditioning.
6. MOST IMPORTANT: After use, rinse/soak in fresh water as soon as you can. If you don't have immediate access to fresh water, drape a towel over the housing and keep in a shaded area. DO NOT LEAVE IN DIRECT SUNLIGHT. Salt water leaves residue that can deteriorate your only protection of the camera: the O-ring. When soak, be sure to press all the buttons of the housing to ensure everything is cleaned out.
Lastly, underwater technique, IMO, is more important than type of camera used. External lighting or use of underwater filters (whether in-camera or preferred external red filter) brings the true colours of sea life out. And a general rule of thumb, get as close as you can (which is why a wider angle is beneficial). Remember, you're shooting through water which is much denser than air, with the addition of misc. sea particles clouding up visibility.
Hope this helps!
Cheers,
Michael
5D Mk II | EF 20 f/2.8 | EF 50 f/1.2L | Yashinon 50 f/1.7 | Samyang 85 f/1.4 | 580EX | 430EX II
Michael Chu