Furlongs are almost always used in measurement on land, analogously to the statute mile. This is the only hydrographical “furlong” in the citations given by the Oxford English Dictionary.  They quote Izaak Walton, The Compleat Angler (1653) :

VIAT. But Master, do not Trouts see us in the night?

PISC. Yes, and hear, and smel too, both then and in the day time, for Gesner observes, the Otter smels a fish forty furlong off him in the water; and that it may be true, is affirmed by Sir Francis Bacon (in the eighth Century of his Natural History) who there proves, that waters may be the Medium of sounds, by demonstrating it thus, That if you knock two stones together very deep under the water, those that stand on a bank neer to that place may hear the noise without any diminution of it by the water.

Readers of this blog are invited to join the Idiosopher in a fellowship of skepticism.