Getting around to nearby dive / snorkelling sites. Use a dive boat. Check out with your hotel / guesthouse. It's easily available. However, you may need to share the boat with other divers / snorkellers.
If travelling 4 hours by boat to an island is fine with you, then Pulau Aur is your next destination. I heard you can get a speed boat there but it's going to cost you a bomb if you do not have the volume.