Coastal mist or fog (3,4)
I believe the answer is:
sea fret
I believe this is a double definition.
'coastal mist' is the first definition.
(I've seen this before)
'fog' is the second definition.
(I've seen this before)
(Other definitions for sea fret that I've seen before include "Mist coming inland at the coast" , "Fog" , "North Sea haar, for instance" , "coastal haze" , "Threat to visibility" .)