Is peninsula the opposite of harbor?

No, dear, its a 'stiky out bit' which extends from the main land into the sea.

No. It is land that juts out into the water, such as Florida does.

I suppose one could take the view that a harbor or bay is the ocean equivalent of a peninsula, and thus in that specific sense, its opposite. It is not an immediate thought though, it requires taking a fairly unusual perspective.

No. Indeed a case could vaguely be made of using "bay" as the opposite to "peninsula", but the case is weak. As with many geographical features, there is no true "opposite" to peninsula.

For that matter, there is no real opposite of "harbour".

No. They are unrelated.

A peninsula is a land mass that is surrounded on three sides by water.

No.

No, peninsula is a strip of land that stretches out. The harbor is where boats dock. I suppose the opposite of harbor is "port".

No.
As other people have suggested .. the opposite would be BAY.

No, it's not.

Sort of. Most harbours are on bays, which is the same shape as a peninsula, just with the water, and land being opposite in each.

No, if you were looking for an opposite, it would be "bay," not "harbor." That is to say that "isthmus" is to "strait" as "peninsula" is to "bay." The reason it's not "harbor" is "harbor" indicates use by ships, a place where shipping traffic is received or at the very least where ships or watercraft can find shelter. "Peninsula" has no such corresponding connotation. A "peninsula" is a tract of land that is bound on three sides by water. A "bay" is a tract of water that is bound on three sides by land. "Bay" has no connotation related to shipping.