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On certain nights around Koh Hong, the water glows. It sounds dramatic because it is. Tiny bioluminescent plankton light up blue-green as you move through the water, every stroke leaves a trail of light.
The Hong Island Krabi team3 min read

Bioluminescence at Koh Hong typically peaks during the warmer months (April to September) when the plankton concentrations are highest, though sightings can occur year-round. The effect is most visible on moonless nights or during the new moon phase.
Our night swim tours launch from Krabi in the early evening and return after dark. The tour includes a guide, life jackets and a briefing on the plankton. No prior swimming experience is required, though you should be comfortable in open water.
⚠️ Bioluminescence is a natural phenomenon and cannot be guaranteed on any given night. A faint display or no visible plankton is not a reason for a refund, because the sunset, the island stops and dinner all still take place. Book the tour for the afternoon and the sunset, and treat the plankton as the bonus.
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See toursNo. The organisms are wild and their concentration in the bay changes from night to night with tides and currents. Dark conditions make a faint glow much easier to see, but no date, moon phase or operator can guarantee a display.
No. A faint display or no visible plankton is not a reason for a refund, because it is a natural phenomenon and every other advertised part of the tour still takes place: the afternoon at the islands, the sunset, dinner and the night swim itself.
The new moon and the few nights around it, because that is when the sky is darkest. A bright full moon washes out a faint glow completely. Cloud cover helps for the same reason. If your dates are flexible, check the moon phase before you pick one.
Yes, within the crew's judgement. You swim from the boat in a lit area wearing a life vest, with the crew in the water alongside. You should be comfortable entering open water, and the crew decide on the night whether the sea is calm enough to swim at all.
Children can join the tour and many do, but anyone entering the water at night should be a confident swimmer and must stay with a parent. Anyone who would rather not swim can watch from the boat and still see the water light up around the swimmers.
Honestly, rarely with a phone. The glow is faint and moves, so it needs a camera that can hold a long exposure and something to steady it. Most guests get a better result by putting the phone away and watching it.