Hi Nick,
Well basically its down to quite a few things. The whole P2 workflow, whilst on paper seems the perfect solution, is also a very pricey way to go. There are a lot of equipment considerations and in a business such as this as you're very aware equipment choices are paramount to performance and efficiency.
The camera is not in any way proven, especially underwater. Sure we've seen clips posted all over the web of ideal condition set ups by experienced filmmakers etc but this is not a camera for the newbie. A lot of people on different forums have obviously bought this camera and then you start getting the "how do I turn it on" posts?? A lot of cash to throw down in anyone's book for what could be a dissastrous learning curve!
My main reasons are basically broken down into the differing areas of practicality:
It will not prove practical to take underwater due to shorter recording time offered by the P2 cards. Larger capacity cards will trickle out eventually but at a premium.
Housing options are required that are far and beyond the norm (I think we've all heard my sentiments on that) and are prohibitively expensive.
Price.....ouch!!
Whilst I firmly believe this kind of technology to be the future I also would prefer a while to wait for the format to be more widely accepted in the NLE world. At the moment only a few suites are able to work directly with the media without having to first import and wrap to other format extensions. It all takes time and could be better and less time consuming, if seemless.
Give it a while, when the HVX350 hits the world in about two years I'm sure by then everything will be more affordable, the image will be a bit sharper and we'll also have affordable options for taking the baby in the water. Including monitors!!! (enough said) 
Options:
I have been looking at the Z1. I did hear about certain panning problems as the camera tries to keep up with the motion and compressing to Mpeg2 at the same time. Any feedback by other members on that???? Also the dropped frame issue. Not too sure how ofetn that could happen but wouldn't it be a bummer if halfway through that Manta birthing sequence at the most critical point the camera decided to drop a frame!!! Ouch and also a very expensive glitch. These were the two main problems. Everything else would be acceptable and I am still looking at the camera.
I am open in my decisions and as such am sniffing around other options.
Cheers to all,
Mark.