Ko Sak is one of the most well-known diving spots in the upper Gulf of Thailand, located within Mu Ko Chumphon National Park. Although it is a small island without sandy beaches for walking like many typical tourist islands, it remains highly popular among both Thai and international divers because of its rich underwater ecosystem and impressive marine biodiversity.The island itself is a limestone formation rising from the sea, surrounded by dark gray cliffs and rugged rocks shaped by years of wind and wave erosion. Some areas resemble natural stone arches and steep rock walls. As boats approach the island, visitors can clearly see the layered limestone formations, while the surrounding sea changes from deep blue to emerald green depending on the sunlight and water depth, creating spectacular scenery for photography.

The true highlight of Ko Sak lies beneath the surface. Around the island are thriving coral reefs, including staghorn corals, brain corals, plate corals, and sea fans spread across underwater rock formations and sloping reef walls. The underwater topography is ideal for scuba diving, offering both shallow areas suitable for beginners and dramatic drop-off zones for experienced divers.Marine life commonly seen around the island includes parrotfish, butterflyfish, damselfish, clownfish living among sea anemones, squid, and small reef shrimp. During certain seasons, larger marine species such as barracuda schools, batfish, and occasionally whale sharks may also appear when sea conditions and plankton levels are favorable.

Another major attraction is the excellent underwater visibility during calm sea conditions, especially between March and May. Visibility can often reach 15–20 meters, making the island suitable for underwater photography as well as Open Water and Advanced Diving courses. Many diving companies in Chumphon use Ko Sak as one of their training and exploration sites.Although there are no restaurants, accommodations, or tourist facilities on the island, this lack of development has helped preserve its natural beauty and peaceful atmosphere. Most visitors travel to Ko Sak as part of a One Day Trip, departing from the mainland in the morning and spending around 1–2 hours diving or snorkeling before continuing to nearby islands such as Koh Ngam Yai, Koh Ngam Noi, or Koh Mattra.With its tranquil surroundings, relatively untouched environment, and vibrant underwater world, Ko Sak is an ideal destination for travelers seeking a more authentic and nature-focused marine experience in Chumphon, especially for diving enthusiasts and lovers of the sea.

• The best time to visit is between March and May, when the sea is calm, the water is clear, and underwater visibility is at its best.

• During the monsoon season, around November to January, boat services may be temporarily suspended depending on weather conditions.

• If traveling by private car, it is recommended to arrive at the pier at least 30–45 minutes before departure time.

• During long holidays and peak tourist seasons, visitors should book boat trips or diving tours in advance, as seats often fill up quickly.