I'm in the middle of the process myself.
Its the state that mandates all the requirements for any waterfront work.
A boatlift installation requires the Permit By Notification (PBN) application; provided the boatlift is:
1) Installed such that no additional boatslip is created by the lift
2) Is removed during the non-boating season
3) Located at least 20' from an abutting property line or the imaginary extension of the property line over the water
4) Installed in a manner which requires no impact that would necessitate further permit action
Visit their website:
www.des.nh.gov/wetlands to obtain their booklet (in .pdf format) which explains the process. There are other applications which will work if the PBN process fails. The application fee is $100, plus whatever fees your town requires to process & forward the application; plus building permit fee.
Allow yourself several months for approval. The required paperwork can be intimidating; you may want to hire a consultant. Be patient, if it isn't approved by one type of application, try again with a different application. In an effort to protect the lakes, the DES will reject and/or deny any application for the slightest reason. Don't despair; you need to work with the system, not against it.
Or, if you get along with your neighbors, and are feeling confident; just install the damn thing. This is what many folks do, and it seems to work best.