Coastal waters in cool, humid areas where fresh water rivers run into the ocean tend to have lower

The correct answer is - salinity.

The salinity of the water depends a lot of the climate conditions over a water body, or part of it, but also if there's a big river nearby or not.

The hotter and more dry it is, the more saline the water will be. The reason for that is that there's a lot of evaporation of the water, thus the salt is concentrating more. This can be easily seen in the Red Sea.

The cooler and wetter it is, the lesser the salinity of the water will be. The reason for that is that the evaporation of the water will be on a low level, sot eh salt will not be able to concentrate a lot. Also, if there's a river that brings in freshwater, than the salinity will drop even further, as there will be water without salt in it coming in and making it rarer.