Larry, those are loaded questions!
First of all, we all have different tolerances for quality. If you look at some of the numbers, the H3 is awesome. 45mbps @ 2.7k/1080p res in a box about the size of a matchbox is pretty impressive, as is the cine color profile and MWB. However, look deeper and you'll find the sensor is VERY small and that means no matter how good the processor is, the noise levels will not rival those of a 1/2"+ sensor HDR. And the MWB, unlike most camcorders, isn't 1 or 2 button pushes.
Don't get me wrong, The HD3 is awesome for the package but only you can decide whether it's good enough. I suggest you check out the quality on your big TV and see the difference. If you are ok with many of the docos and in car POV type shows on TV, the most likely you will like the GoPro. Since you dive though, the noise levels aren't great.
As for the FX, are you talking about the FX7/1000? Those are HDV and shoot at 1440x1080. The resolution is probably slightly above the GoPro's MPEG4 compression due to the slightly bigger sensor size. But they are huge and unwieldy and the housings even bigger. And they use HDV tape, which is a pain. The good thing is there are old 2nd hand systems lying around.
I do believe XR series is still widely covered by housing manufacturers. They don't give the prettiest picture but better in someways than a GoPro HD3. There's also the Canon XF series where housings are available as well. If you are shooting video, I'd get a proper video camera vs a DSLR. DSLR are nice but the resolution is not nearly as good as a camcorder.