It is a fact, though, that swell height is often reduced considerably when in a large harbor/port. Due to surrounding structures, harbor/port depth, piers, and other physical obstacles, wave length and height is often reduced. Of course, the wider the mouth of the harbor/marina/port and the less deep it is, and the more ocean facing exposure, the greater the waves will remain similar to the ocean swells outside.
In ports such as hamburg and rotterdam, where there is a long, and often more calm, water way feeding the harbor, the water is noticable calmer than open sea. Even ports like San Francisco, of which I am from, you can notice large wave behavior changes as soon as you leave the mouth of the bay, just west of the Golden Gate bridge. Again, a lot of this has to do with the geography of the area . . .
And as evidence your statement is true, look at Dover for example, the water would be considerablly too rough if it were not for the barriers at the outer edge of the harbor, however, Dover Harbor is not very deep in terms of distance from entrance of the harbor to the furthest point. I could go on and on regarding this, but I will stop.
However, for the water being calmer in harbors, blame me. :p
