The Hudson River is an estuary: a freshwater river that flows into the ocean, where it mixes with salt water. The degree of mixture between fresh and salt water is called the salinity gradient. On the Hudson, the salinity gradient at a given location can vary considerably depending on factors such as time of year, tidal flow, and precipitation. It's a different story on the Connecticut River, and no coincidence that Dutch explorers dubbed it the Fresh River; so much water flows out through it that the river's waters are fresh little more than a mile inland from Long Island Sound.
Salinity levels have an immediate effect on the surrounding ecology, since many aquatic creatures can survive only in either "salt" or "fresh" water. It's just as important for sailors (of any era), who need a fresh water supply to survive.