First off, I don't think there is anything wrong with the P7000. On land it is a bit dated in size, but that should not be an issue underwater.
The Fantasea case seems nice, but it is a non-round port case, so add on lens will be an issue. But if you are taking images that don't need an add on lens, then what difference does it make?
Canon is so successful in underwater camera's, because they make a line of housings that are very affordable (not the depth rating of the Fantasea, but similar in shape.) Ikelite also makes housing (next step up) and then there are the expensive cases (but nice).
Once someone starts with a camera line, they tend to stay with that line.
Image quality wise, they should be very similar.
Having a threaded front is nice because you can add macro and wide angle, but they cost, and good wide angle costs a lot.
While the P7000 has the same wide angle capabilites as the G12, so there is no advantage there.
The Fantasea does have the "big eye" for wide angle, but my experience with that has not been that good.
The canon, with say the Ikelite case, can use any of half a dozen or so wideangle/fisheye add on lens.
If you like shooting the type of images that the flat port can provide...I would call it "object shooting" (they have some very nice examples on their website). The choice would seem to be the canon and the canon housing versus the P7000 and the Fantasea... I believe the canon is way cheaper in that case.
Plan to add lens, then the canon with any of the cases that take 67mm theads would be a good idea.
Want to dive deep, then the canon housing is out.
I am sure there will be no shortage of viewpoints on this.
Note: If you dive a lot, and do it fairly deep (say 100 ft), the canon case button springs will wear out and the camera will not work...it does not leak, you just cannot take a picture until one is up around 90 ft. Ok, that took 3 or 4 hundred dives, but I still hold that against them.