In reply to the sheep:
Snorkelling. In Cornwall. In May. Are you completely mad? It'll be bleeding freezing.
However. That time of year, the most consistent snorkelling conditions are going to be around Lamorna Cove, so if you look at the stretch between there and Porthcurno, you won't go too far wrong.
In terms of accommodation, you've two main choices for camping: Treverven, over the top of St Loy, or Treen, near Porthcurno. Treverven's a nice, quiet spot, but not so close to a beach as such; Treen's within easy reach of the Porthcurno strip - and more importantly, right next to one of the best pubs in Cornwall - the Logan Rock Inn. In terms of accommodation within bricks and mortar, check out t'internet - May isn't a massively busy time, although the weather is often cracking in the second half, so you may get some good deals in the area. There are a good few rental places in Lamorna and, despite the distance from a decent pub, this would probably be your ideal spot.
Oh, and a quick message from the wife (a diver): if she or any if her mates spot you in the water with a spear gun, she/they'll be delighted to introduce you to the art of body piercing. Just the messenger, you understand
Martin