It's only arbitrarily easy since water has a density of 1 kg/l in metric, as it was designed to do so.

it was designed to do so

That's also what we mean

I will also maintain that when describing human temperatures for weather Fahrenheit is a superior scale

That's one where I'd say it's almost entirely personal preference. Water freezing being at zero is handy because it's nice to know when there's ice/snow outside, but that's also something you could just learn to remember in Fahrenheit too. Doesn't really matter with that system. Kelvin and Celsius being bros is nice but it's extremely rare that I use that so eh.