I don't think you will need any special cover its only snorkelling.
I insure with this company they cover 40 adventure sports as standard, have a look at link there is a table of whats covered. I have used them for last 2 years but never made a claim. I took their black policy out last time for about £120-£130ish, they had a lot cheaper options http://www.insureandgo.com/travel-insurance/Annual-Travel-Insurance.htm
