Diving in Koh Tao and Samui allows you to see some amazing stuff like Christmas Tree Worms (Spirobranchus giganteus).

They are Christmas tree shaped tube-dwelling worms with twin spirals of plumes used for feeding and respiration, common dive sites around Koh Tao and Nang Yuan Islands there are you can see them are Japanese Garden, Red Rock and
These worms are sedentary, meaning that once they find a place they like, they don’t move much.
They come in many colors including red, orange, yellow, blue and white and though they are small with an average 3.8 cm in span, they are easily spotted due to their shape, beauty and color always makes an eye-catching display.
Christmas Tree Worms are found on coral reefs in tropical waters worldwide, in relatively shallow waters less than 30 m.
Christmas Tree Worms are polychaete feeders that feed using their radioles, or «feathers» that circle outward from the central spine, to catch phytoplankton floating by in the water.
Plumes are also used for respiration. Though the plumes are visible, most of these worms are anchored in their burrows that they bore into live calcareous coral. Christmas Tree Worms are very sensitive to disturbances and will rapidly retract into their burrows at the slightest touch or passing shadow.
For more knowledge about it, you can take PADI Fish Identification course.

Little underwater beauty on Koh Tao, Thailand: Christmas Tree Worms make dive site Twins so colorful