As others have shared Chileno Bay does not have a sand beach, rather it is made of small pebbles. The advantage of this for snorkeling is that you will not have sand stuck all over you and it does not limit visibility in the water. The downside it can hurt the feet a bit.
We were there on a Saturday afternoon in January as the friendly lifeguards suggested to us that low tide is the best time to snorkel. There was plenty of parking available and places to sit on the beach. There are portable toilets available.
The downside as others have mentioned is that condos and a large resort are being built around the bay/cove, so there is constant construction noise and I am assuming also eventual impact to the ecosystem with runoff. The other issue is that this is where the snorkel trip boats come from Cabo, so the gas smell and noise from the bigger boats is definitely not calming. :-)
The snorkeling . . . the positive is that this is a protected bay. I felt absolutely no current or undertow of any form while in the water and entry and exit from the shore is easy. The water is incredibly clear and there were an abundance of fish. Some of the coral still looked somewhat healthy and had not been destroyed. That being said I am used to snorkeling in Hawaii were the fish are not fed and there are sea turtles and other sea creatures. From an ocean condition standpoint it is more difficult to snorkel in Hawaii given the currents and waves there and the deeper water.
A major downside is that the fish are fed, so they swarm you and are constantly touching you and blocking the ability to just float and enjoy. It was creepy at times. The water temperature was a bit chilly, but it was January. Probably about 74 degrees and I wore a rashguard.
We had a car, so we were only a 15-minute trek to Chileno Bay from our condo near SJDC and we had our own masks and snorkels. We did not even need fins given the water conditions. Easy to swim to and from shore. Many families were playing in the water without issue. If you have access to a kayak this would be a fabulous protected place to paddle. Make sure to bring your own snacks and drinks if spending an extended amount of time.
Would I pay to take a snorkel boat trip there, no. The boats stayed about 30-45 minutes. However, if you have transportation it is a great little place to go on your own and explore or to escape the chaos of the resort areas.