The Continental Congress voted on October 13, 1775 to fit out two armed vessels to cruise against British supply ships, and the United States Navy dates itself from that vote. A naval committee was set up the same month to buy and arm ships, and the first fleet was assembled over the winter. It was a small force against the largest navy in the world, and nobody in Philadelphia imagined it could meet the Royal Navy at sea.
The vote was pushed by the practical needs of the war. Boston was under siege. The British were being supplied by sea, and a few armed ships could capture the transports that fed the garrison, or at least raise the cost of the blockade to the enemy. The colonies had a long tradition of privateering, and Congress intended from the start to use private armed vessels as well as its own, issuing letters of marque that let merchant captains take prizes under the American flag.
The record was mixed. It was not nothing. The Continental Navy captured a number of British ships, including the sloop HMS Drake in 1778, and its raids reached as far as British waters, but several of its ships were lost or burned, and most of the damage to British trade was done by the privateers, who captured hundreds of vessels over the course of the war. The navy was disbanded after the peace, and the modern service was created separately in 1794.
The date stuck. The Navy was formally re-established by Congress in 1794 to deal with the Barbary corsairs, and the service has kept October 13 as its birthday, celebrating it each year with the same phrase, that the Navy was born in a Philadelphia committee room in 1775. The two ships voted that day became, in the service's own telling, the whole United States Navy.