Hartlepool used to be pretty close until they reclaimed land nearby - must be pretty close between them and Blackpool for second spot.
From memory, Sunderlands old Roker Park ground was pretty close, or it seemed that way on my couple of visits there.
In non league grounds I remember going to Canvey Islands ground when they were in the Conference, a few years back. Their ground is literally yards from the Sea/estuary.