The shore has two moving parts
Finding a former shoreline underwater is the beginning of an explanation, not the end. The sea may have risen. The land may have sunk. Both can happen together. NASA calls the height of the ocean against a particular coast relative sea level, and includes vertical land motion among its causes.
Ocean warming changes water volume. Currents redistribute water. Sediments compact, tectonic boundaries move, and land can continue adjusting after the removal of ancient ice sheets. These processes do not operate at the same rate everywhere.
Sources: s1
Even the ocean surface is uneven
NASA's account of ocean topography describes a sea surface shaped by gravity as well as circulation. The geoid is a reference surface reflecting gravity and Earth's rotation; actual ocean height also responds to currents and other influences. A water surface on a map is therefore not a perfectly uniform ruler wrapped around the planet.
Sources: s2
Questions worth asking at a submerged site
The age of an object and the age of its flooding need separate evidence. A useful investigation would seek a dated shoreline indicator, evidence that it has remained in place, a stated height reference and a local reconstruction of land and water movement. A row of stones at one depth cannot, by itself, establish when it was built or whether a single catastrophe submerged it. This distinction matters when comparing an actual coastal site with a legendary lost city.