Tides in marginal seas are tides affected by their location in semi-enclosed areas along the margins of continents and differ from tides in the open oceans.  Tides are water level variations caused by the gravitational interaction between the Moon, the Sun and the Earth. The resulting tidal force is a secondary effect of gravity: it is the difference between the actual gravitational force and the centrifugal force. While the centrifugal force is constant across the Earth, the gravitational force is dependent on the distance between the two bodies and is therefore not constant across the Earth. The tidal force is thus the difference between these two forces on each location on the Earth.
In an idealized situation, assuming a planet with no landmasses (an aqua planet), the tidal force would result in two tidal bulges on opposite sides of the earth. This is called the equilibrium tide. However, due to global and local ocean responses different tidal patterns are generated. The complicated ocean responses are the result of the continental barriers, resonance due to the shape of the ocean basin, the tidal waves impossibility to keep up with the Moons tracking, the Coriolis acceleration and the elastic response of the solid earth.
In addition, when the tide arrives in the shallow seas it interacts with the sea floor which leads to the deformation of the tidal wave. As a results, tides in shallow waters tend to be larger, of shorter wavelength, and possibly nonlinear relative to tides in the deep ocean.