I've been scuba diving a while and have also done a fair bit of freediving. The scuba diving got me into underwater photography with a housed SLR. However, I only take my housing when I go on scuba - Just for the sheer bulk of the gear, the strobe arms etc. I just cannot think how I'd get reasonable photos. A point & shoot is different, I used to have a Canon Powershot S30 in its WP-DC300 plastic housing. It had no strobes attached and it handled really well when snorkelling or freediving.
If you really want to take a housed SLR while freediving, then I'd guess you're limited to wideangle. Macro shots take too much time to set up. And when freediving, the ONE thing you haven't got is time (unless you're called Umberto and have grown gills :-)
I'd look for a setup that allows you good natural light photographs - as strobe arms cause too much drag in the water.
So look out for a fast wideangle lens and find a matching port. I would suggest the Sigma 14mm f2.8 EX or maybe even the 10.5mm Nikkor fisheye. You'll get good shots in natural light with those lenses and not need much time to set anything up while underwater.
I don't want to keep from using a full setup while freediving, and I don't have much experience with my housing while freediving myself, it's just what I think would make sense.
Good luck.
Fred