Actually there is a difference between a snorkeler and a skindiver. The snorkeler remains at the surface the entire time. They remain a snorkeler until they submerge on a breath hold dive. Snorkelers that go below the surface of the water are called skin divers. There is also more of a distinction being made nowadays between skindiving and freediving. People are starting to consider freediving more of an extreme form of skindiving.
We offer a great Snorkelpedia for your enjoyment.