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Assuming the levels fell at a noticeable rate year-on-year, how long would it take the tides to be a significant (200m or so) distance from where they used to be? How long until the new land would be useful? How much would trans-oceanic trade be affected?
(Sorry I know this is a lot)
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)...
the Netherlands has spent much of the last century reclaiming land from the ocean
While it's not a natural phenomenon, The Netherlands has been working to reclaim land and protect against flooding for centuries. The projects are called Delta Works and Zuiderzee Works.
Both projects have been seen as extremely successful in both protecting existing cities and lands and allowing new cities to be built such as Flevoland, which has significant green space.
Assuming you can haul in a layer of top-soil, the land could be used for farming as soon as it dries out. Since it's now prime beach-front real-estate, you'll probably have fancy hotels and condo's lining it. We currently can build on the beach no problem, just drill down to bedrock. Land use would be pretty much immediate in highly populated areas where there is now more prime real-estate.
As for seaports and trade, cities would be mildly annoyed their dock is now a dry dock. Once the sea-level lowering is known, they could dam a river feeding the ocean and use a Lock to raise the boat to the river level. Just like the Panama Canal.
Speaking of the Panama Canal, they'd lose out big time if the ocean receded enough to empty it, which 200m would probably do. Things like that would probably be the biggest change to trans-Atlantic shipping. Small countries that count on shipping and port revenue would be left behind as the shore-line shifted.
Larger countries would just make their port's modular and equipment disassemblable. This is already the case with cranes for skyscrapers. So the effects to them would probably be minimal.
nothing would change for humanity in terms of biology.
because just this was the state of the world during the last ice age, and humanity survived that no problem.
it also would give us about 25% new land to work with, and would give you the means to walk all the way to Russia from Alaska.
