The tide was a long way out, and that is when this beach is best. All that wet sand turns into a mirror, and the people left on it get a second version underneath them, so every figure comes doubled.
The light was going and the weather was turning, both at once, which is the recipe for the good stuff. Someone stood out on the flats waving something over his head at nobody in particular. Then a full rainbow came up over the whole bay, faint but complete, the way they do right before the rain arrives to prove them right.
All on the 55, dragged to ISO 6400 to hold any shutter at all in that light. You do not plan a rainbow; you just happen to still be out on the sand with the camera when the sky decides to hand you one.