There is a two-hour window at each end of the day when the Blue Lagoon looks like the photographs. Everything else is a compromise.
The short answer
Be anchored before 10:30, or after 16:30. In July and August the basin holds thirty-odd boats between 11:00 and 15:00; outside those hours it is often nearly empty. Nothing else you can control — month, weather, which operator you pick — matters nearly as much as the hour.
Month by month
May: sea around 19–21 °C, almost no boats, and the clearest water of the year. Bring a layer for the ride out. June: 22–24 °C and still quiet — this is the month we would pick if we were paying. July and August: 25–27 °C, warm, lively, and busy from mid-morning. September: 24–25 °C, the crowd thins after the first week, and the light gets noticeably better. October: 21–23 °C, quiet, and the most underrated month on this coast.
Why the colour changes
The turquoise comes from pale sand at one to four metres reflecting light back up through the water. That means it depends on sun angle, not on water quality. Overcast days look grey-green no matter the season, and midday sun flattens it. Low, side-on light in the morning and late afternoon is what produces the colour people expect.
What we actually do
Every full-day itinerary we run leaves Trogir at 09:00 and is anchored in the lagoon before the fleet arrives. On the long Šolta day we come back past it late in the afternoon once it has emptied. It is not a clever trick — it is just the only way to see the place properly.