WASHINGTON (AP) _ The Supreme Court ruled today that Long Island, N.Y., though surrounded by water, is legally part of mainland New York.

As a result, the court said that Block Island Sound is an internal state waterway and Rhode Island and New York may continue to require ships using the sound to have state-licensed pilots.

By a 9-0 vote, the court ruled against the federal government, which said Long Island should be considered an island legally and Block Island Sound part of the high seas and thus not subject to state regulation.

″The general rule is that islands may not normally be considered extensions of the mainland for purposes of creating the headlands of bays,″ said Justice Harry A. Blackmun in his opinion for the court. But ″in this case, Long Island functions as an extension of the mainland, forming the southern headland″ of Block Island Sound, he added.

Rhode Island and New York officials filed suit in the mid-1970s to determine control over the sound, 172 square miles of waterway on the nation’s east coast.

In effect, the court ruled today that Long Island is a peninsula that is part of New York state even though it is separated from the mainland by water.

The court upheld a ruling by senior U.S. District Judge Walter E. Hoffman, who was appointed by the court as a special master to hear evidence in the case.

Hoffman said that Long Island Sound and Block Island Sound west of a line between Montauk Point, Long Island, and Watch Hill Point, R.I., constitute a bay under an international treaty adopted by the United States.

By treating Long Island as a peninsula, the court’s ruling today encloses the waters of Block Island Sound, qualifying it as a bay subject to state regulation rather than exclusive federal regulation.

New York officials argued that Long Island qualifies as part of the mainland because of its large size - 120 miles long and 12 to 23 miles wide - and the relative narrowness of the East River, which separates the island from the New York mainland.

They also noted that Long Island contains four New York counties, including the Borough of Brooklyn, as evidence of its close ties with mainland New York.

But the federal government said that since the East River is navigable and relatively deep, Long Island should be considered an island.

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The court rejected a proposal by state officials to widen Block Island Sound by extending the border of the bay in an easterly direction.

Under Rhode Island and New York laws, foreign ships and U.S. vessels registered for foreign trade must take on a state-licensed pilot to help them navigate Block Island Sound.