A more thorough explanation:

Arm of the Sea

An arm of the sea is a part of a river or bay where the tide goes in and out. It can go far into the land where the water of the river is pushed back by the tide.

One example of an arm of the sea is the Hudson River in New York. The tide goes in and out of the river, and it goes far into the land. Another example is the Bay of Fundy in Canada, where the tide can rise up to 50 feet.

These examples illustrate how an arm of the sea is a part of a river or bay where the tide goes in and out. The Hudson River and the Bay of Fundy both have tides that go far into the land, which is a characteristic of an arm of the sea.