Why not a C172 on floats?
Mr Faure says: "The floatplane version of the 172 has a useful load of only 634 pounds, 359 pounds less than the landplane. Three of the FAA's theoretical 170-pound adults weigh a total of 510 pounds, leaving 124 pounds of useful load for fuel, or about 20 gallons. That's assuming the three people aren't taking anything heavier than a wallet apiece, of course."
An amphibian version would be worse of course.