British Army slang in use before the Second World War, a
sand rat is a whore, hooker or prostitute in a
seaside town. They became known as
sand rats because, like rats, they only came out after dark and would take their clients down to the beach for sex on the sand.
A British
squaddie going on leave might say to his mates, "Right, I've got me 'water wings and calamine' and I'm off to play with the
sand rats."
Note
Water wings and calamine referred to condoms and barrier cream issued to soldiers to try to reduce the risk of them contracting STD.