The Buttermilk Channel is a small tidal strait, approximately one mile long and one-fourth of a mile wide, separating Governors Island from Brooklyn in Upper New York Bay. At one time this channel could be crossed at low tide, and farmers would bring their cows over to Governor's Island for grazing.
Historically it has been a busy shipping lane and the most convenient access to the Brooklyn waterfront.
February-April 1806, The Medical Repository, pg. 433:
The passage between Governor's-Island and Long-Island, formerly called Butter-milk channel, and within the memory of man, both narrow and shallow, is now eight fathoms deep.
September 1815, The American Magazine, Pg. 173:
She lost one of her wheels and anchors, and was driven into the bay below Governor's Island, in danger of foundering. At day-light, however, she contrived to beat into Buttermilk Channel, and fortunately got into Whitehall.