Viz'art

Pam, The Aquatica close up lenses uses a quick snap mount, it can be installed and removed underwater within second

I think the Nikon D7000 is one of the best balance of performance found on camera to bring underwater, you can get a high quality system without having your wallet cleaned up, the choice DX format lenses are well suited for this type of photography (I feel DX has a better selection of lenses than FX has, and getting the same coverage from fisheye to macro, is bound to be substantially less expensive than it would for a full frame system.

Canon versus Nikon: in your case,  with Nikon, your Ikelite strobe can be used in TTL with the Ikelite 4302 or 4301 external TTL converter, it can only be used in manual with a Canon

Only downside I see to the D7000 is that it is not as sturdy built as I would like it to be, but then again, it is not meant to be used by pro photographers or cavemen. and it is solid enough for pretty much all users who care about their gears.

Lens choice, Tokina 10-17mm Fisheye Zoom, either a Nikon 10-24mm or Sigma 10-20mm, Nikon  AF-S 60mm f/2.8 Macro or/and Nikon 105mm f/2.8 VR, I also use a Sigma 17-70mm on occasion, it has moderate wide angle coverage and a fairly good close up performance and it is, IMO inexpensive.

Alastair, A 4" being a smaller radius dome than the 8", you will need to stop down to get the edges sharp, most of the time, I pretty much shoot the Tokina 10-17mm behind the 4" dome port, I still carry the 8" for splits shots and for shooting super wide lenses, but I could live without it if I had too.