[Ammi]

..I think there are quite a few places/destinations where snorkelling and other activities are available...maybe google 'activity holidays' and hopefully it'll give you a good choice of different costs/distances etc....

..the Canary Islands in general are quite good, I think...we went to Fuerteventura, probably when the boys were at a similar age to your son and the place that we stayed had just about everything..snorkling, scuba diving, tennis, water polo, volleyball, archery, football...lots of other stuff, I can't remember it all but your son would love it, I think...