Hapuna Beach, a little north of the Marriott, is nice for snorkling if the beach is sparsely populated and you can even spot a whale or two.  However, what is surprisingly good is their snack bar, which serves a good array of sandwiches and drinks.  The smoothies are a must, especially after some time in the water.  Since there is nothing to eat in the immediate vacinity, this place is a goldmine!