Crab and lobster are my favourite things to hunt, followed by big plaice. It's a fun way to dive, and it keeps you fit as you're always swimming, ducking under ledges, and scouring gullies.
Here is a bit of info I wrote about four years ago, about hunting lobsters:
Lobsters are naturally curious creatures, and will normally come out of thier holes a bit to investigate strange goings on. Lobster holes can be literally anywhere. We found an 8lber under a piece of fibreglass in the middle of a mud flat once, so it pays to leave the fish and just dedicate your dive to hunting bugs.
What it comes down to, is a lot of stealth. If that fails, gentle persuasion and then bullying.
When you find a likely hole, your intended victim will normally be near the opening, so you will be able to see its antennae and/or claws. Don't dive right to the lobsters front as it will back off quickly. Approch from the side if possible, and go slowly. If you can check for an escape at the back of the hole do it now, as thats where the lobster will do his vanishing act from. Put your dive buddy here if you have one. If not, block it off carefully with a stone.
Approch the hole again from the side, and create a small disturbance in the sand a foot or so in front of Sebastian. He should move forward to have a look-see. Now is the time for the "Hero Grab". Come in from behind, do it quick, and not not hesitate in the slightest. Grab him on the carapace where he can't nip you and hold him firmly, as he will be flapping all over the place. Do not grab the claws, as before you know it, thats all you'll have left.
If you missed, Sebastian will bolt back in his hole. You can now do two things. Persuade him out or bully him out.
If your buddy is at the escape tunnel, try and make the lobster flee its hole. Give it a few pokes with your spear (not too hard). Your attentive buddy can then grab him as he makes his escape.
If you're on your own, you might have to now bully it out; meaning, make it a bit unpleasent so it has to flee it's hole from the front entrance. This can work, just as long as the hole isn't deep. If it is a proper deep hole, he will just retreat and have a snooze. Unload your spear, and use the barb as a hook. Lobsters hate things behind them, so try and get the barb behind its tail, give it a wiggle, and it should turn around. Then again, it might not. Its trial and error at this point, just keep on trying to move him. Try not to kick up too much silt, as if you do, you might not see the lobster as he escapes right under your nose.
It comes down to lots of practice. I find it quite fun, and like the challenge posed by a deep holed lobster. If you fail, he will normally be there next time to try your luck again, unless you tell one of your buddies and they get there before you.