Tidal waters are usually synonymous …
With some degree of saltiness.
At Naples Beach, where the tidal range is under three feet, you can usually count on 35 parts per thousand worth of sodium. Or in other words, salty to the taste. Just a mile inland the canals are fresh at under 1 part per thousand. In between, the water is what we call brackish.
Estuaries are where fresh and salt waters mix. How much they mix is usually a combination of the seasonal influx of freshwater — either by direct rain, ground water discharge or a river or creek — and the twice-daily waxing and waning of low and high tides.
Saltwater intrusion is the term given to situations where canals, over-pumping or other diversionary drainage works open the door for more saltwater to infiltrate into the naturally freshwater zone. Saltwater intrusion can be solved with engineering to a degree, including saltwater barriers, moving or reducing ground water pumpage and other water works.
Too much freshwater can pose a problem for estuaries, too, such as at the mouth of the Caloosahatchee River. Once a meandering coastal creek, over the past 150 years it has been lengthened and straightened into a sizeable canal. Today, the Caloosahatchee Canal (C-43) serves as the main release valve for spilling flood waters from the Big Lake to the coast, often overwhelming the downstream seagrasses and bivalves and harming the estuaries overall health.
Other times, freshwater will not drain after a storm because high tidal waters causes it to back up. This effect can be especially pronounced during tropical storms that major deluges coincide with high tide stands.
In short, tidal waters are interesting from a number of standpoints, both for their seasonal and daily changes in their salinity, flow direction and magnitude.