I'm a Canon fanboy, so take this with a grain of salt:
I currently shoot a G11. From the few UW shots I've seen, and from the specs, I would guess the G15 would be an excellent UW compact, especially for macro and close-in photography. It looks like it can do true 1:1 macro shots out of the box, meaning that for macro, only the new Panasonic DMC-LX7 can compete unless you start buying wet lenses.
Both the LX7 and the Sony RX-100 seem to offer better video options and specs, but I've read about WB and focus issues for these cameras. The G15 (from what I read) seems to do better in these areas (or at least equal), but you
will be limited to 24fps at 1080p. If you're just posting video to the web, then it probably won't matter, as last I checked (admittedly a while ago) both youtube and vimeo downsample to 24fps anyway. If you want 60fps, the G15 isn't your camera.
Wide-angle is the mystery. Inon has released an adjustable wide-angle wet lens
UWL-S100 ZM80 that reports to compensate for the longer zoom that is standard on the G-series cameras. I've only seen a handful of photos taken with the lens, and I've contacted Reef Photo and Backscatter to get more samples, but it's new on the market, and it will apparently take a few more weeks for folks to start sharing their photos. If the lens works the way it's supposed to, then that will make the G15 the camera to have (for me at least).
The LX7 and the RX-100 are easier to get wide-angle wet lenses for, and I've seen some really good wide photos from the cameras. The Sony has serious issues with close-in and macro photography, that make it a deal-killer for me, but it has a huge sensor, and a ton of video options not present on the Canon. The LX7 will focus about as close as the Canon, will take standard wide-angle wet lenses, and has more video options than the Canon. It would be my second-choice, and may end up my first if the Canon won't perform with the new Inon wide lens.
I've used two models of the Canon factory housings. I was happy with both, and it looks like they've improved a bit in the current generation, but they're useless below 130ft, where the pressure ends up depressing a button or two, effectively disabling the camera functions. That's a deal killer for me, as I'm spending more time at or below 130ft with a camera, and it's pointless to take a camera that won't work. I'm currently salivating over the Nauticam housings. I've used their housings before, and they're impressive.
Also, have you looked at the Olympus 4/3 cameras? You can have one for very cheap if you go with the factory housings. They may be worth consideration if you're in the market for a new system.