The level of the sea on Earth has risen an average of about 8 inches since early in the last century. A more substantial rise is forecast for the present century. There are three reasons the sea level rises. The reason easiest to understand is the increase of water. When ice not presently in the sea melts, that volume of water is added to the sea, causing it to rise.
Another reason is thermal expansion. As the temperature of the ocean rises, it expands. About half of the rise since the 1800s is due to thermal expansion. Because the ocean has not been warming uniformly, the ocean level has come up more in some places than in others. Changing ocean currents are further capable of forcing water levels higher in some areas. Of all the concerns about the changing climate, the rise in sea level is likely to be the most costly to mitigate.