I deal with waves all the time and have worked with ocean swimmers for years on how to get through and deal with waves. I've also been a diver for most of my years on this earth and have taught snorkling in the past, particularly in socal waters.
I was particularly surprised when in Hawaii to see the cattle boats with 100 - 200 people going out to a spot to snorkle and just how low the bar is. On the trip last April off Kona I was required to have a floatation vest (insurance rules) (SIGH), and saw lots of people swimming around with foam noodles under their arms to aid in floatation as well as those using boogie boards with port holes in them to peer underwater.
Frankly THIS IS NOT what the diving community would call snorkling skills, but rather adaptations to get the masses to experience some very pretty underwater sites. I think this is fine, but I wouldn't call this snorkling.
However, given that this seems to be the normal in tourist operations, with a little practice in a pool or non-wave environment, your parents could do very well. The main skill to learn is not to breath through the flooded tube (snorkle) and how to clear it. It can become an unconcious, reflexive action but usually takes time to become sensitve to the resistive pressure when breathing and to differentiate between a flooded tube and a clear one. It also requires discipline and technique. I recall breathing in quite a bit of water learning it and still occassionally inhale when all is not clear. (cough)
BELIVE IT OR NOT, YOU CAN BREATH THROUGH A FLOODED SNORKLE TUBE, but you must do it very, very slowly. If you find yourself with time and are in a comfortable setting, you can try this but again it takes practice and should be in a depth you can stand in. Remember, breathing in water is also the definition of drowning!
I personally found it annoying that I was required to wear additional floatation gear because the ability level of people they take into the water is largely so poor. (Wow I'm grumpy this morning!!!)