I agree with Matthew regarding the goals and your existing experience with cameras even above water.
You can go point-and-shoot with the S95 and the G11/12 and get fantastic results provided you learn your settings with a variable fixed lens plus adapters and get to spend a lot of time playing with light.
If you're the type who looks to control more settings, such as changing your lenses (and your domes which can cost a whole lot as your setup grows), then you'll definitely want a dslr setup, second hand or otherwise.
Most of the people I've spoken to who are heavily into U/W photography have started with a point-and-shoot. Then they eventually "graduated" to the larger rigs of the dslr world as their capabilities started feeling constrained by point-and-shoot rigs.
To follow Mike Barrass, my half bar.
I've used Sealife DC series with their flashes and got great results, also in the relative dark murky conditions here in Denmark, the DC1400 is a fantastic 14mp camera rated to 60meters and shoots video in HD. Together with the new videolight they have is a good bet. Let me know if you have trouble getting hold of them. We sell them in the shop aswell. sealife-cameras.com
take a look at http://www.uwphotographyguide.com/
Generally speaking, given the lake conditions (green water, low-light, limited macro opportunities, etc.), you might consider adopting black&white UW photography, to take hard-hitting pictures. For this you need a wide-angle lense.
DSLR or compact? Given that you do not have luggage weight issue as when flying, I recommend the former, though costs are more than double for a whole set (camera, wide-angle lens and housing), but if you opt for B/W photography you do not need flash lights: maximum aperture, maybe a tripod, and you can shoot terrific photos.
Olympus 5050 will offer all you need in order to learn the basics...Full manual control most importantly...Cheap and good solution for every beginner... See the shots in my profile for your reference... :::---)))))