Yes, the water at WPC is not really swimmable, so obviously nobody snorkels there. However, there are places nearby, like the Salt Pond beach. FWIW, there are few hotels in Hawaii where one is likely to successfully snorkel off the hotel's own beach. The Maui Sheraton is one of the few that I can think of.

I find the comments about "more to do for kids on Maui" to be interesting. Since there are more people on Maui, there certainly are more activities. That said, most of the additional activities are quite expensive (almost all organized activities in Hawaii are), and I frankly doubt most visitors pay these prices for young children to participate. Indeed, other than whale watching (which can be quite affordable and enjoyable for the younger set), I don't recall ever paying for "activities" for my kids on Maui. If you wanted to spend the money, I'm sure you could also find plenty of organized activities on Kauai to keep the kids "entertained" -- although, frankly, I think they would be just as happy on the beach.