The Sea Turtle Conservancy (STC) gives two reasons for keeping beaches dark at night when sea turtles are laying or hatching.
The first reason the STC gives is that a nesting turtle likes quiet, dark beaches. If she cannot find such a spot after several attempts, she will choose a less than ideal spot to lay her eggs, and perhaps even lay them in the ocean. This reduces the probability of survival for the hatchlings.
Having been liberally splattered with sand by a turtle laying in broad daylight (Aldabra Atoll, the Seychelles) as one of a ring of observing tourists (quiet and motionless tourists), I personally put greater emphasis on the second reason given by the STC.
Lighting near the shore also can cause hatchlings to become
disoriented and wander inland, where they often die of dehydration or
predation. Hatchlings, scientists believe, have an innate instinct
that leads them in the brightest direction, which is normally
moonlight reflecting off of the ocean. Excess lighting from the
nearshore buildings and streets draw hatchlings toward land, where
they may be eaten, run over, or drown in swimming pools.
I'd welcome more expert opinion on the sensitivity of nesting females to light. The adult turtles I have encountered seem phlegmatic, but maybe inside they are nervous. Also, I am perhaps reading too much into the behavior of the Aldabra Atoll turtle. Aldabra Atoll has only a small research station and very few visitors -- it is as unlike a Florida seaside community as any place on Earth.