I'm at the Hilton Curacao right now. The beaches are tiny, some rocks, some white sand. But we found Porto Marie in Westpunt to have a very nice beach for more white sands than rocks. It has a nice beach cafe which overlooks the beach.

There are some lovely beaches at the WEST END. Klein Knip is one and then every 1/4 to 1/2 mile another. During the week, not during school vacations you will find them quiet and the snorkeling along the boulders that form the bays very good. Actually better than Bonaire for snorkeling.
You need a car but it is worth renting a Hertz car for $50/day one for 2-3 days for you likely will want to go back if you enjoy snorkeling.
I met a fellow at Santa Cruz Beach #1 that has 2 speedboats to take up to 6 people to 3-4 excellent spots not accessible by the beaches.
He is a character. He charges $10-$15/person for up to 3 hours and takes you to one spot and comes back to pick you up and on to another spot. I suggest you tell him you want 30-35 minutes tops at each if not he will come back in 50-55 minutes and that is too much at each. He has life preservers and some equipment but it is best if you rent the mask/fins/snorkel from the hotel. He serves a great lunch on the dock for $12-$15/person including a beer.The East End has one good spot near the Sea Acquarium Resort but does not compare with the West End.