Is it okay for my wife that is 20+ weeks pregnant to go snorkelling? Anyone have good advise?

Snorkeling is fine as is swimming as long as she doesn't over exert herself and the Gynae says it is fine. But definitely no diving because of the depth and water pressure. If your wife has been quite active before and during pregnancy and so far no complications and no signs of fatigue and breathlessness it should generally be fine and even be a stress buster for her :) however as snorkeling expose her to the sun try not to do it in midday and remember to ask her to take fluids breaks.