Do seaplane have designated landing areas? Do seaplanes have to land in particular places or are they allowed to land pretty much anywhere there is room to land safely?
Taken from seaplanes.org:

While most pilots assume the FAA has jurisdiction over landing areas, including water-based landing areas, the truth is much more complex. Jurisdiction rests with the person or organization that &quot;owns&quot; the waterway. This may be a Federal or state agency, a local government, a private corporation, or an individual. Determining who controls a waterway is the first step in determining whether it is legal to land on that body of water.
A second complication enters in the picture with overriding jurisdictions, most commonly state-imposed seaplane base licensing requirements. In several states, notably Ohio, New Jersey and Indiana, seaplanes may not land unless the proposed landing area is certified as a seaplane base, regardless of whether the waterway owner provides permission or not. To determine whether this is an issue in your area, call your state aeronautics office (often a division of the state department of transportation), check the Water Landing Directory, or call SPA Headquarters (863/701-7979).