Lake George was a lake on Earth.

In 2373, after encountering and defeating an alien lifeform, which had tried to coax her spirit away from her body, Kathryn Janeway wanted to celebrate this victory together with Chakotay by drinking champagne while sailing on Lake George in the moonshine. (VOY: "Coda")

It was not specified which Lake George she intended to sail on. There are at least eight bodies of water called 'Lake George' on Earth, from Australia to Canada. The Star Trek Encyclopedia (4th ed., vol. 1, p. 452) states that the reference is to the Lake George in northeastern New York.
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