As others have said, get it weighed, but I would pay for the paperwork and get it noted on the paperwork, what the boat is loaded with (eg full tank of fuel, tackle gear, etc). Keep it in the glovebox and it may help with coppers who ask the question. If the QT guys are there, they may still put you over a weighbridge.

I did this with a large horsefloat once and got it weighedwith my two horses and camp gear in it. Paperwork noted the two horses by their registration numbers and camp gear. Three times I was questioned by the cops who wanted to send me off to a weigh station, but I was able to pull out the paperwork and was left to get on with my day.

Bit of a pain, but I thought it was worth getting right in the first place.

Cheers
Steve