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 "owns" 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).
From what i understand yes.
They land on waterways
NEW HAMPSHIRE AERONAUTICS ACT
Prohibitions and Penalties
422:27 Seaplanes in Operation on Public Waters. –
I. All seaplanes shall be considered boats while in operation on the waters of the state and shall be subject to the marine rules of navigation, except that they shall be exempt from all laws and rules concerning the operation of boats for the purpose of landing and taking off from such public waters.
II. The operation of seaplanes shall be subject to any restrictions placed upon the use of public waters by rules adopted by the department of safety or the department of environmental services.
Source. 2002, 6:1, eff. July 1, 2002.
If you are using your plane as a boat you could effect another view. Such as fishing and bring a citation from Fish and Game or Marine patrol. The NH RSA. is clear any state water body, May be used for take off and landing without being subject to additional boating regulations. Additional restrictions are hard to see from the air , but apparently can exist. By local town ordnance and state management areas.
On another note as of 2018 it seem NH fish and Game has made changes to the state wide boating access FIS 1600 . Restricting the use of their controlled public launch and parking No person shall land or launch and aircraft or drone from.