I have not yet pulled a boat with my bike but I have pulled loads of the weight range you are talking about and even considerably more. For pulling a trailer made to be pulled by a truck or car with a bike the thing to use is a two wheeled pup trailer with a ball hitch behind the bike to take the tongue weight of the trailer and also have a place to mount a 12V battery and electric trailer brake controller. Anything more then a couple hundred pounds of weight on the trailer and you need trailer brakes otherwise applying the bike brakes especially on a downhill will just jack-knife your bike in front of the trailer and the trailer will drag or run over the smaller lighter bike in front of it.

You can make such a pup trailer to fit your bike by re-bending and welding onto the handle of a cheap hand cart used for moving trailers around on a level surface by hand like this:



With some clever wiring and a variable pot you can rig up the electric trailer brake controller to your rear brake cable, set the trailer brakes to engage before the rear brake on the bike does.