There are few snorkel tours that go to Los Arcos or the Marietta islands. The snorkeling is just OK. There are no reefs like the east side of Mexico in order to have great snorkeling. There is nothing much to see snorkeling just off shore. If you’ve snorkeled in other places, it will be disappointing IMO.
I used to bring a mask and snorkel and head to a beach just to the north of La Cruz called Playa Manzanilla. The sheltered cove is good for swimming and maybe the closest area to Bucerias for some visibility in the ocean.
Seen lots of neat fish swimming around.