Cold water offshore cools the air above it, a shallow deck of cloud forms over the ocean overnight, and the sea breeze pushes it inland across the coastal plain. Locals call the worst of it June Gloom, and it is at its most persistent in late May and June.
It is shallow — typically a thousand to two thousand feet thick. That has two consequences for a helicopter ride Los Angeles operators fly in the morning. Under it, the coast is grey and flat. Above it, you are looking at the tops of clouds instead of the city. Neither is what you booked.
It also burns off. By late morning on most days the deck retreats to the shoreline and the basin is clear. If you are flying in May or June, take a slot at ten or eleven rather than eight, and prefer an inland route over a coastal one.