I'm not sure what you consider decent wide angle. But of course a DSLR will have lenses available that have a wider field of view - and cost a ton more. Only you can decide what's worthwhile.
The G12 is a nice camera, but it's lens is not especially wide angle (28mm eq), and that gets worse underwater due to physics. In air, the 28mm lens has a FOV of 75 degrees, but in salt water through a flat port that becomes a FOV of 54 degrees. Not good news, but you can't fight physics.
The G12 is a poor candidate for adding a wide angle wet lens to for another reason -- because of the way in which the G12 lens moves when you zoom. At it's widest, the lens is not fully extended in the port, that is, it's recessed back from the port window. That means the port has to have a relatively large piece of glass to avoid vignetting, and any wet lens to cover that glass area must be large -- which means big, heavy and expensive. In fact, because of this, most wet lenses will vignette at widest angle on a G12 -- they have to be backed off to say 35mm eq to avoid cropped corners. So because of the large lens port area, the G12 is a poor candidate for adding a wet lens for wide angle photography. Other cameras with smaller ports work out better - such as the Canon S90 or S90. You may notice that most of the wide angle buffs here use the Canon S90/95 instead of G class.
But if you still like the G12, what's available? There are different options for different housings. And there are two types of wet lenses available: air dome, or glass.
Air dome: This is the least expensive and simplest. It gives a modest improvement to field of view, roughly undoing the loss of FOV caused by water. With the G12 in a Canon DC34 housing, the Air Dome enhances the FOV to roughly 80 degrees, not bad. It has a domed front element, air inside, and a flat rear element, with a hood for protection. There are different mounts for different housings, either push on or screw on to fit the housing port. Cost is about $220. Below is a thumbnail of the Fantasea Air Dome on Canon housing. Ikelite makes a similar air dome, WD-4, for its G12 housing.
Glass lenses: These are better lenses than the Air Dome, made of glass not plastic and having more elements (typically 4 groups, 5 elements). These are much heavier and more expensive than Air Domes, but enhance the FOV much more. They come in various mounts for various camera housings, but few to fit a G12 housing.
Because of their weight, it is risky to mount these to a plastic housing like the Canon DC-34. I don't know of a commercial adapter available to do it, but you can cobble one from a step ring. I would not advise it for a heavy glass lens -- use for macro wet lenses only. Illustrated in thumbnail #3 below.
Canon G9 & G7 housing - DIY 67mm lens adapter - Wetpixel :: Underwater Photography Forums
If you really want a glass wide lens, go to a metal housing, like the FIX G12 Housing or 10Bar G12 Housing with the wide lens (these are not wet mount). The FIX is pictured in the thumbnail below. For more info on these housings, see:
Fix G12 Housing for Canon G12 - Fisheye Fix Housings & Accessories - Optical Ocean Sales Underwater Photo - 800-359-1295!
10Bar Underwater Housings - Optical Ocean Sales Underwater Photo - 800-359-1295!
