Koh Ku Du Yai

Ko Ku Du Yai is a small, uninhabited limestone island in Phang Nga Bay, reached only by longtail charter or kayak from Koh Yao Noi — there’s no ferry service and no fee to land. Two large limestone stacks, with a smaller rock between them, guard the entrance to a shallow green lagoon on the island’s east side, which is the reason most boat operators stop here at all. A second, smaller island — Ko Ku Du Noi — sits just to the west.

Getting inside the lagoon means paddling through a narrow tidal passage cut into the rock. Check tide times with your boatman: the passage floods at high water and narrows enough that only a kayak or a confident swimmer gets through comfortably. Once inside, the lagoon is calm and shallow, ringed by sheer limestone walls hung with stalactites and small trees rooted into the cliff face. Outside the lagoon, the coastline alternates between pebble beach and rocky shelf, and snorkelling off the east-facing shore turns up reef fish over patchy coral, with visibility that depends on the tide and recent weather.

Insider Tip: Book the lagoon entry for mid or low tide rather than high tide — at full high water the cave mouth can be too shallow, or too rough, to paddle through safely.

There’s no jetty, no shade structure, and no facilities of any kind on the island, so bring your own water, reef-safe sunscreen, and snacks. The dry season, November to April, gives the calmest crossing from Koh Yao Noi and the clearest water for snorkelling; expect a choppier, shorter operating window during the June-October monsoon.

Watch out: No regular boats stop here — you need to charter a longtail from Koh Yao Noi or join a kayaking tour that includes it, often alongside Koh Hong and other lagoon islands nearby.

Ko Ku Du Yai itself carries no entrance fee. If your route also takes in Ao Phangnga National Park sites elsewhere in the bay — Tham Lot or Khao Phing Kan, for instance — that park’s own fee, 300 THB adult / 150 THB child for foreigners and 60 THB / 30 THB for Thai nationals, usually gets folded into the tour price rather than charged separately on the day.