In my experience, personally-maintained, non-standard markers are common for near-shore hazards (e.g. bleach bottles, plastic duck buoys).
I'm no expert in the law, but I find nothing prohibiting you from doing what you described, safely:
http://www.gencourt.state.nh.us/rsa/...270/270-26.htm says you can't interfere with formal nav aids, nor create hazards to navigation. Section b says you also can't use haz-mat containers, unless you've neutralized them.
If you look at the regulations on swim rafts (
http://www.gencourt.state.nh.us/rsa/...0/270-72-d.htm) it sounds like the state is fine with the landowner adding floating objects, again, if they're not a hazard to navigation. Maybe you can even add your name to the proposed marker/float and call it a swim raft to make it expressly legal? I don't see a size definition on what constitutes a swim platform.