Scuba diving or snorkeling is not available on Penang Island. But you always have the option of excursion for enjoying these water sports. Ask your hotel to arrange a diving excursion for you. Best place for any diving or snorkeling excursion is Pulau Payar Marine Park off Langkawi.
You would need full day for Pulau Payar trip. The excursion package would include lunch. You would take a couple of hours to reach Pulau Payar from Langkawi.One is not allowed to stay at Pulau Payar because it is a conserved area. So you have to stay at nearby islands and have day excursion to Pulau Payar.
You can also visit Gua Tempurung limestone caves, two hours off Penang.