There are certain marginal seas with their entrance more or less enclosed. For example, the Mediterranean Sea is connected with the Atlantic Ocean by the narrow Straits of Gibraltar.
When there is high tide in the open ocean outside the Straits of Gibraltar, there is an inflow of water in the Mediterranean; but this cannot be sufficient to cause tidal rise and fall in so large a sea.
Therefore there is practically no tide in this sea excepting a very small one generated in this sea itself. The reason for the small tides in the Mediterranean Sea can be found in the narrowness of the entrance through the Strait of Gibraltar.
On the contrary, a more open bay, like the Gulf of Mexico, is less isolated, but even in this bay the Atlantic Ocean tide causes only slight rise and fall. However, in each case powerful tidal currents flow through the narrow opening due to the difference in the water lever of the outer ocean and the partly enclosed sea.