What is the boiling point of water at sea level in Celsius?

A hundred, and every word of the qualifier matters. Boiling is not a fixed property of water; it is the temperature at which its vapour pressure equals the pressure pushing down on it. Reduce that pressure and water boils cooler.

On top of Everest water boils at about 70 degrees, which is why food genuinely will not cook properly at altitude - the water is boiling vigorously and is simply not hot enough to do the chemistry. Denver has a whole set of altitude-adjusted baking instructions for the same reason. A pressure cooker runs the argument backwards, holding the water liquid past 100 so things cook faster and hotter.

Take the pressure low enough and water boils at room temperature, and lower still and it boils and freezes simultaneously - the triple point, at about 0.01 degrees and a six-hundredth of atmospheric pressure, where all three states coexist. That point was so reproducible that it was used to define the kelvin until 2019, when the definition was moved onto a fixed physical constant instead.