Why water

Because it’s one of the most abundant resources available on earth that is imperative to our understanding of the existence of life and is the commonest solution regularly used by humans in scenarios where temperature is a relevant factor.

why sea level?

Because vast swathes of humanity live at or near sea level, where we already arbitrarily nominate the level of atmospheric pressure to be 1 ATM.

While K has a natural zero-point, the gradation (by your logic) is entirely arbitrary as well because it’s based on C and not some other cosmological constant.