If the bunkhouse was built in the 80's then it would be grandfathered under the Shoreland Protection law regardless of use. Wetlands would not have jurisdiction over the structure unless it was built over the lake, in the bank of the lake, or in a wetland. If it's located over the bank or the lake itself; that's a significant problem. I'm going to guess it is not built over a wetland but if it is, I have seen that permitted by DES before. Your problem is going to be related to the septic system and whether the additional loading for the bathroom in the bunkhouse was ever permitted. I believe that so long as the building is plumbed it needs to be discharging to a septic system even if someone were to argue that the bunkhouse did not have a "bedroom". NH law requires that the sellers of shorefront property complete a site assessment evaluating the septic system for disclosure to potential buyers. Ask to see it. As for reliable info on the grandfathering dates on septic systems go right the the Subsurface Systems Bureau at DES at (603) 271-3501.
As for the docks, 20 years old puts the construction date in the 90's. Definately not grandfathered. Dates for "grandfathering" are 1969 for permanent structures and commercial seasonal structures, and 1978 for all other seasonal structures. Look for a permit, odds are it has one. Technically the permit should be recorded at the registry of deeds and should come up in a title search but if not then try the online permit query found at the link below. It will query records back through October 1987.
http://www2.des.state.nh.us/OneStop/...its_Query.aspx