We keep our boat in Fells Point most of the year, where the water changes with the seasons. I'm not just talking about freezing and thawing, the type of water changes drastically throughout the year. We are located quite a ways of the river, and the water will fluctuate its salinity with the seasons.
Typically, the water will go fresh in the winter and salty in the summer, with periods of brackish in the transitional months. While a hydrometer is a cheap instrument that can be used to measure the salinity of the water, simply observing the aquatic life will also provide a rough estimate of the waters salinity.
Sea Nettles like water with a salinity around 10 to 16 PSU. In the winter, the salinity goes very low, and in the peak of summer, the salinity is very high. When we see the sea nettle populations boom in the early summer, we know that the water is becoming saltier, and is in their happy range of 10 to 16 PSU. Once the sea nettles disappear, we know that the salinity has exceeded their happy range and is much saltier. In the fall, the sea nettles return as the water is once again passing through their happy range of salinity. When the sea nettles disappear once more in the winter, we know that the water has now become fresh again.
It isn't the most scientific of methods, but it does help quench the quick curiosity of "Is the water fresh or salty?"
All it takes is a quick glance at the water as you walk along the pier!