While most float "planes" will ask your weight, I have not heard of them charging an additional charge if you are over 250 lbs., like the helicopter excursions.
I have done float plane tours when I was over 250 lbs. and was not charged an additional amount. The weight balance calculation is not as tricky in a float plane as it is in a helicopter. It does have an effect as to how many people can be in the plane.
It does make the flight more comfortable if you are under 250 lbs. There isn't a lot of room in a Cessna 185.