A "shallow sea" refers to an area of the ocean that is not very deep. It is a relatively shallow body of water with a depth that is not too far from the surface. Full definition
They may be a kind of sun block, protecting corals from intense light in shallow seas. (sciencenewsforstudents.org)
Extensive continental regions were flooded by shallow seas ranging in water depth from a few to little more than 100 metres (330 feet). (britannica.com)
While researchers aren't exactly sure why warm water masses are rising into shallow seas, the point is that it could be a major contributor to sea level rise. (natureworldnews.com)