Krabi 7 islands sunset tour: the cheapest boat, and the one that finishes in the dark

The plankton are the reason to take an evening trip rather than a morning one. Certain plankton give off light when the water around them is disturbed, so when you swim the movement of your arms leaves a trail of blue-green sparks.

It needs darkness. That is why these trips finish late rather than at sunset — the swim happens once the light has properly gone, usually from the boat at anchor.

It is not guaranteed. How much you see varies with the season, the moon and the water. A dark, moonless night shows far more than a bright one. Reviews reflect that spread, which is part of why this trip sits at 4.4 rather than higher.

The sunset itself. The hour before dark on the Andaman coast, with limestone stacks in silhouette, is worth the trip on its own even if the plankton are quiet.

The four-island trip runs in daylight around Poda, Chicken, Tup and Mor, usually from mid-morning, and the sandbar at Tup is its centrepiece.

The seven-island trip runs in the afternoon and evening. More stops, less time at each, a barbecue and the plankton swim. The islands overlap partly with the four but the shape of the day is completely different.

If you only do one: do the four islands in a private longtail at $105 if the budget allows, because the sandbar and the control of the day are worth it. Take the sunset run at $53 if it does not — you get the best light of the day and something the daylight trips cannot offer.

If you do two: four islands by day, seven islands at sunset. They do not tread on each other.