There seems to be not much agreement about what this sea-gown is. Depending on the source, it is a sailor’s tunic, a nightshirt worn at sea or the fog.
I like that there might be special clothes for sea voyages and that they’re called gowns. I also like the specificity of how Hamlet is wearing this gown. Scarf-d about him suggests to me he’s either tied the gown on with a scarf – like belted it or just sort of tossed it around his neck. It gives him a sort of cavalier feeling. What does he care for actually fastening his clothes? He can just scarf them on, just drape them over himself as he gropes in the dark, like a guy with his jacket draped over his arm, ready to fly if he has to.