Ahem, I don't think Cunningham was attempting to detail ideal sailing conditions, but rather comparing a life on the sea to life ashore, which for most folk meant travelling no farther than a few villages away. From this standpoint, a wet sheet and a flowing sea make perfect sense as symbols of sailing life--you immediately know the setting in vivid, rousing language. Here, the wet sheet in two short words suggests hauling the lines amid storms or spray--suggesting that sailing life ain't pleasure cruising--while the flowing sea implies both travel and the great might of the ocean. You might as well complain about "bounding mains"--they're not jackrabbits.

It's well-established that the song's a romanticized, fake sea song written by a landlubber. But if you want to earn erudition brownie points, stop quibbling about nautical points taken out of context and instead tell us about the tune.