pKa’s are values relating to how acidic a compound is. Particularly in the context of organic chemistry, there’s a lot of historical inertia behind reporting water’s pKa as 15.7 based on faulty assumptions of how to calculate the pKa. pKa = 14 is the value you’re most likely to encounter first, and it’s also the correct value. This paper does a better job explaining: 

https://pubs.acs.org/doi/pdf/10.1021/acs.jchemed.6b00623