Since land is involved and land is valuable, it turns out that measuring sea level has been an important measurement for a long time. Apparently even with satellites etc., we are not that much better at it now than in the 1800's
In fact, the accuracy of the monthly mean sea level observations—which most climate scientists use as a starting data point when looking at long-term ocean trends—as calculated using measurements from the microwave sensors is almost exactly the same as what scientists calculated using tide gauges in the 1800s, Bushnell says. That’s a testament to the painstaking, detail-obsessed work done by generations of tide gauge observers, technicians taking care of the equipment, and NOAA analysts putting together the results from all the nation’s gauges every month, according to Gill.
If you look at that data, last century the mean sea level rise is about 1.7mm per year and more recently appears to be about 3.2 mm per year. Lots of planning is going on, and many cities have serious issues and will have to spend a lot of money....
In your case, where the water level is dropping noticeably you could take a look at the Aral Sea which in a lifetime has changed dramatically and is considered an environmental disaster.
By the late 1980s the lake had lost more than half the volume of its pre-1960 water. The salt and mineral content of the lake rose drastically because of that, making the water unfit for drinking purposes and killing off the once-abundant supplies of sturgeon, carp, barbel, roach, and other fishes in the lake. The fishing industry along the Aral Sea was thus virtually destroyed. The ports of Aral in the northeast and Mŭynoq in the south were now far from the lake’s shore. A partial depopulation of the areas along the lake’s former shoreline ensued. The contraction of the Aral Sea also made the local climate noticeably harsher, with more-extreme winter and summer temperatures.
In your world building case, and if it is ocean levels, there are a couple of things to probably consider that are dependent on the specific conditions. In sandy areas, hurricanes and weather can result in pretty dramatic changes such as barrier islands and the reshaping the islands after a storm. People still build in those areas. But in terms of making the land useful, even if the area being uncovered is more silty "salt" can the salt tolerance of plants would be an important factor. But going from a river or a bay to a tidal marsh, to some kind of meadow doesn't necessarily take geological time, but could occur over a couple of lifetimes, or in special situations perhaps shorter or longer.
To give an idea of how much area could be impacted the East Coast of North America extended more than 100 miles into the Atlantic during the last ice age. Doggerland is probably an even better example connecting Denmark and England. That was a sea level change of about 400 feet (120 meters)...