Why don’t you just plot the data as log(Rw) vs. log(Sal) with a linear regression line through the data? Non-linear fitting is fine, but log-log is easier.
y = ax^b so log(y) = log(a) + b log(x)
find b from your regression.
Alternatively, looking at your data, it’s pretty obvious that the product of Sal x Rw is a constant (power is minus unity). So you could also plot 1/Rw vs. Sal. That would give a nice regression line but tell you about the value of a only (because b is fixed to -1).
EDIT: if you STILL want to model the data as is, then I suggest reading this. Personally, I’d log the data. A search on “Non-linear Regression R example” turned this up.