Sadly good diving is the one thing missing from Goa.
There are a couple of places in Calangute now, one behind Little Italy off Holiday street. One on Titos road I think.
I'm a qualified BSAC diver, and although I've been to Goa about 15 times now I've never bothered diving there.
I've been snorkelling a few times and the viz is cr*p to say the least.
Think it's down to all the dredging that goes on in Goa.
There's a place on Bogmalo beach you could try. Our diving officer went with them once, but said it wasn't upto much.
Obviously it's probably going to be better than UK, but I wouldn't expect too much.
I used to speak to a lad who was one of the instructors at the Titos road, he said it was very hit and miss. I think
he was doing it mainly to get his dive numbers up to qualify as advanced instructor.
Apart from that you'll love Goa, save the diving for Sharm in Egypt. Can't be beaten. Or of course the Maldives.
Have fun
Barry