There are two beaches that can be easily reached: the one in front of the resort, where the boat will drop you off, and the one on the opposite side of the island, that can be reached in a 5 minutes easy walk.
The first one is long and nice and ideally located. The second one is smaller and, in my opinion, really beautiful and pristine: no facilities of any kind, perfect sand and sea, lush vegetation around.
Probably there are other small beaches around Ko Kradan, but I didn't have the chance to find them.
Snorkelling is excellent around Ko Kradan. The coral is lively and the water is usually very clear. No need to charter a boat as it's possible to easily swim to some interesting spots directly from the beaches.
Snorkelling around Ko Kradan is easy also for beginners, as there are colourful fishes and corals aven at just a couple of meters of depth.
If you like snorkelling, don't forget to bring your mask with you (and snorkel & fins, if you like; I don't). It was impossible to rent one on the island.
Lovely view. The sky getting darker and I saw a tourist building fire. I'm sure it would be nice to stay there long enough to see nature drown into darkness.
A good visit to Emerald Cave at Ko Muk. Go snorkeling at some of the nice spot where you can see plenty of fishes and corals. And lunch (fried rice) on boat.