Re: Salinity provides a good proxy for both rainfall and evaporation,
"We end up with lots of interpretation based on very little direct evidence"
Welcome to experimental physics! I know it seems like "bad science" when scientists rely on proxies, primarily because it can be hard to understand why and how proxies are used, but all of the things we can "see" directly have been observed already, so the newer things we look at have to be viewed indirectly. Ergo proxies, whether that be new particle discoveries, galactic interactions, climate, or something else.
I find it odd that if ppl who have a political view on climate modelling/change read a story based on proxy measurements they decry them as inaccurate, but then "ooh" and "ahhh" over a new particle discovery which has been discovered by measuring proxies (not referring to the OP, just in general).
Salinity isn't a bad proxy btw - the salts have a tendancy to stay where they are, there's a finite amount of water in the water cycle which has a finite number of states, so measuring the salinity gives you a decent picture of where the water is at any given time.