Fully enclosed seas tend to vary in water level both season to season and over longer timescales (e.g. the Aral, Caspian seas.) As shallow as your sea would be that means significant changes in salinity, unless maybe it's fresh water all the time.

I suspect that some intelligent creatures would engage in dredging channels and reclaiming land for farming. There might be unintelligent creatures which build up their own structures (especially if the water's fresh), along the lines of beaver dams or anthills. The dolphin-equivalents would be troubled by both kinds but it was only recently that the Yangtze river dolphin went extinct so they can probably tolerate some.

Fortifications and borders inside the sea would be difficult; you could get a seagoing Khan or Napoleon conquering the whole thing in his lifetime. The Romans held most of the Mediterranean for a long time. Or a faster-growing people (in D&D worlds humans, orcs or goblins) expanding out over the top of slower-growing peoples. Are there powers which would stop this happening? You were wanting a diverse range of cultures here.