What level of precision are you looking for? It all comes down to the the accuracy and precision of the scales (load cells) you're using.

A high quality scale might have an accuracy of +/- .1%. If it's intended for a 5000lb race car, that would be +/- 5lb. Maybe that's good enough for you, maybe it isn't. I suspect a truck scale would not provide a sufficiently accurate reading for a 400lb catamaran. Same goes with using a couple bathroom scales. They may get you in the ballpark, but if you're looking to get within say +/- a pound, they're probably not going to cut it.

McMaster Carr has 1000lb capacity dial scales good to +/- 5lb for $1k, or a digital version good to +/- 1lb for $1800 just to put things in perspective.

Sailors love to talk about weighing boats, but without an understanding of the accuracy of the measuring equipment, the data is basically useless.

sm