Now I think you will find that it depends on what type of 172 .... and whether it has an STC that might also raise the MTOW - eg Penn Yann 190HP Conversion.
I am part-owner of a 172 Reims Rocket floatplane (similar to Hawk XP). 210HP 6-cylinder CS-prop. On wheels 1553lbs empty, 947lb useful load.
MTOW is 2500lbs
On straight floats (EDO2440Bs) she has basic empty weight of 1768lbs = 732lbs useful load. Floats weigh 340lbs, landgear weighs 125lb.
(Full tanks (39imp.gal) + 2 POB plus 124lbs) or 3 POB and 220lbs fuel (30gal)
Later 172 models have MTOW of 2550lbs giving 782lbs useful load (extra 7 gallons).
There are only two types of amphib floats available for the 172, and we are in the middle of the process of obtaining EASA certification for one of them
The amphibs are 100lbs heavier than the straights. We reckon to have therefore 632lb useful on amphibs with possibility of raising that further (adding STC'd STOL kit).
There is a 180HP SuperCub out at Como operating on amphibs (I-BUFF)
--------------
PS If anyone is interested in flying floats in Scotland or becoming a shareholder in the amphib (then you can fly solo
) do send me a message.