There is no fresh water to salt water "line". It is one continuous gradation. "Fresh" is consdidered less than 0.5 parts per thousand (ppt) of salt. Full srenght sea water is considered about 30 to 35 ppt. Anything between 0.5 ppt and 25 ppt is called brackish. Most of the Ches Bay falls into the brackish category, with the southeastern part of the bay being the saltiest, and the northern part being the freshest.
The Bay's salinity is highest at its mouth, where water comes in from the Atlantic. As you head north, salinity gradually decreases. On a salinity contour map, the lines of equal salinity (isohalines) tend to run in a SW to NE direction. Most of the fresh water enters the Bay from the northern and western rivers such as the Susquehanna, Potomac, Rappahanosck, Patuxent, York, and James Rivers. Rivers on the east side like the Sassy, the Chester, and the Wye can be wide rivers at their mouths, but they have relatively little flow, ad don't add much in the way of fresh water compared to the monsters like the Susqueahnna and Potomac. This causes the west side of the bay to remai9n fresher than the corresponding eastern side. Also, the earth's rotation, which causes the "Coriolis effect", causes flowing water in the Northern Hemisphere to deflect to the right, which means that saltier water moving up the Bay veers towards the Eastern Shore. Therefore, water near the Eastern Shore is saltier than water on the western side of the Bay, even at the same distance up the bay. So, as you go due east from Annapolis to Kent Island, it theoretically gets saltier. So, head north and west for fresher; south and east for ssltier.
Here's a good way to know you're in water that's pretty salty. Do splashes leave salty spots when the water dries? If you can, launch in an area that is fairly fresh. This way, even if you venture in to salt, the traveling back through the fresh on the way to the ramp will "flush" the engine. I usually don't risk it and I just flush anyway.
Have a fun trip.
