The 0 in Fahrenheit was based on nothing and the 100F was supposed to be human temperature but it is off by some degrees
The water is not an arbitrary temperature, the weather is water dependant, at 0C the water will freeze and you get snow/ice instead of rain
0°F is when the ocean freezes
100° F was human body temperature, later revised somewhat with better measurements and a decrease of parasites . The average person in those days in London had a slightly higher body temperature than today
0F is not ocean freezing, is the freezing temp of a brine mix that he chose arbitrarily (some think that he chose that temp because it was close to the coldest his town had ever been and he used it to calibrate the scales of his thermometers)
FYI, the ocean freezes at around 28F
