[sunfish]

I've been waiting to read advice from more experienced people than myself but here's mine, though I'm no expert.
First off, just find an area of rock with holes and cracks and just keep looking. If you keep dives fairly short while looking for good crevices, your surface times will be shorter.
Reefs close to shore are ok and I think the inside (less exposed) is better.
When you're on the surface and see a vertical crack in the rocks, maybe a metre or more wide, swim down it shining your torch in. These usually get narrower and eventually close up a few times, making a series of little dens for crabs and lobsters. Lobsters seem to prefer the lower ones but crabs like any.
This could be a really useful thread so come on and get posting lads.