The colour of a lagoon comes from two things working together: the white sandy bottom reflects sunlight upward, and the shallow depth means that light passes all the way through and bounces back. In Ha'apai the water can glow in shades of turquoise, aquamarine, and pale blue-green - colours that are almost impossible to describe until you have seen them.
Because the lagoons are sheltered from big ocean waves by the outer reef, the water inside is calm and warm - usually around 27 or 28 degrees Celsius, about the same temperature as a comfortably warm bath. Fish, rays, and sea turtles move slowly through the shallow water, easy to spot from above.
Many of the Ha'apai islands are ringed by wide, sandy beaches with no buildings, no roads, and no noise except the wind and the sound of the sea. Visiting here feels a little like being the first person to ever arrive on a tropical island - peaceful, beautiful, and completely unhurried.