Read the description of a caisson light - the cylinder goes from the surface of the sea to the floor of the ocean. If a light is built on top of a breakwater it is not likely that there is a cylinder going thru the breakwater to the floor of the ocean/ sea/ bay/ whatever.

John's second link is the same as my second link. The NPS site, as well as the CG site, provide pretty good descriptions of the various construction types used in building lighthouses.

Candace Clifford's listing of lighthouses is an excellent resource, one that I have often used when trying to find information on various lights.

Dave