We did not dive in Malta, only on Gozo (scuba).
However, we did one marvellous dive (non scuba) in xlendi bay. The bay itself is rather shallow, I couldn't find a nice spot below 11 meter. What was really fun was the rather shallow cave there at the right hand side of the bay (when you're on the 'boulevard'). When you swim out, is is quite hard to find the entrance directly. However, a local dive master (forgot his name, connected to Moby dives) pointed out that if you first swim around the cliff, and swim back, it is very easy. I have to admit that I could not resist to say hi to the scubas that were coming out of the cave at 10 meter...
The cave starts out above water, and then slopes down, until you have to go under water. Before this you have a small arch where you can choose to stay surfaced, or go down below. When the ceiling hits the water, you can actually see the other side very brightly. I'd have to estimate both the length and the depth of the last bit. Say 10 meter long, 4 meter deep. It did feel longer that the guy had promised... As I've had never been in an overhead environment before (with just a snorkel), I got a kick out of it. But the length and depth was so limited, that it did not in any way feel uncomfortable.
So if you are in the vicinity, I would say, give it a try. I would have loved to do the gozo blue hole, but the wind made the chance of injury too big when we were there without the scuba gear. With scuba it was fun as well.
I'll try to attach a few fotographs I took there - one of my girlfriend on her way towards the tiny arch, and one where she is barely visible going for the exit.
On a general holiday note: I found the atmosphere very relaxing, but that may also have been caused by the fact that there weren't that many tourists yet.