Does salinity affect seawater's pH?
Yes, salinity affects the pH of seawater.  

See RECOMMENDATONS FOR THE
DETERMINATION OF pH IN SEA WATER
AND ESTUARINE WATERS Pure &amp; Appl. Chem., Vol. 60, No. 6, pp. 865-870, 1988.  

An approximation of pH as a function of temperature and salinity is given:  

pH = (2559.7 + 4.5 S)/T - 0.5523 - 0.01391 S  

Where "T" is temperature in kelvins and "S" is salinity in units of parts per thousand.   

As seen above, in the temperature range of Earth's oceans, increasing salinity increases pH.  

See also pH of seawater Marine Chemistry Volume 126, September 2011, pages 89-96.  

Fig. 2 plots pH versus salinity at 25 degrees C and 0.000334 atmospheres of CO2 and shows pH increasing significantly with salinity.