HiZenn:
If you're going to Phuket, you'll have to send a day to visit the islands...I just came back from Phuket last week.
My hubby and I loved those daytrips around the islands like Maya Bay (Leornado di Caprio's The Beach was shot there), Phi Phi Don, Phi Phi Ley, Hin Klang, Monkey Beach, Viking Cave, Pileh Cove, Khai Nok Island ...
For just THB1,200 (about S$50) per pax, you get pickup from your hotel to the pier; snorkelling equipment; 6-course Thai lunch + free flow of coffee/tea on Phi Phi Don which has been partially rebuilt after Tsunami, and free flow of drinks & snacks/tropical fruits onboard. Optional: You can pay extra THB50 for every pair of flippers if you find treading water tiring.
Like Alhana has described, you'll feel like a mermaid swimming in a large turquoise aquarium populated by colourful fishes and sharks if currents are not strong. You can feed the fishes bread and the monkeys bananas. Don't forget to take pictures underneath the waters with a disposable waterproof camera like the one I got
The last stop is Khai Island, only discovered less than 10 years ago. We were given 2 hours to laze on the beach, enjoy free drinks and fruits, snorkel, swim or simply, ride the waves.
If you're going to Krabi, there'll be another set of tours to other islands like Yao Yai, Hong Island, Chicken Island, James Bond Island, etc.
You can sign up for fishing tours to catch marlins too!
good Luck!