Sandra - The A710is is a good camera, but doesn't get you raw support. Of the cameras under discussion, only the long-discontinued S70 allows that. The A710is is also not available yet
The S80 is closest thing you can find right now that has it all, IMO. It has plenty of MPs, a wide 28mm lens, and the underwater mode that you spoke of earlier on.
There is a lot of confusion about the capabilities of Canon camera lines, in part because of Canon's naming scheme. The A series and S series cameras both have full manual controls. The SD series (Different from the S series) do not. All current Canon P&S cameras have the underwater scene mode.
I will note that you can take fantastic shots with the underwater scene mode, and simply not worry about white balance. The scene mode is a compromise setting, as manually setting is the only true way to nail the WB setting appropriate for that depth and ambient light level, but it really does a pretty good job at getting you there. You can't use the manual controls and the UW scene mode at the same time, however, with any camera.
RAW brings it's own set of challenges, despite the WB advantages. The slower write times are due to the monstrous file sizes, and lengthy transfer times, and far lower number of photos per chip due to those huge sizes. I am not a fan, personally, and it sounds overkill for your purposes.
The main differences between the S series and the A series are lithium batteries vs. AA, and a smaller size. The S70 and S80 also have the wide angle lens built in, which is very nice to have. The OEM Canon housing for the S80 supports adding on Inon lenses, which is another bonus... no larger Ikelite housing needed.
For the type of shots you have described, IMO the best camera for you is probably the S80, which can still be found new if you work a little. (Try eBay... there are power sellers that have new ones still.) The OEM housing is nice and compact, there are lens options, you can use it with a strobe by adding a tray and arms later (check out
www.ulcs.com for the best arms, btw), but it also works quite nicely with just the stock housing alone, for bulk-free shots using only the base lens. There are some excellent A series cameras currently for sale as well, but there are issues with add-on lens availability if you don't get the Ikelite case, and they do not have a wide angle lens.