Koh Tao is the most popular island for divers far and wide. The deep water around (around 30m mark) ensures exceptionally good visibility and a rich range of marine life, including turtles, which is why there feels like there are more dive schools than bars on this island.
The longest and most developed beach on the island with a variety of restaurants, coffee shops, diving centers and bars is Sairee Beach. But there are nicer beaches, a bit hidden but easily accessible with a private yacht.
Koh Tao (Turtle Island) definitely belongs to the best islands near Phuket for boat trips.