Must all topographic maps have a contour line that represents 0 meters of elevation Explain?

Not all topographic maps need a contour line that represents sea level because on some topographic maps you can analyze, that the contour line does not start on sea level. There are several reasons for this. One reason maybe that, the mountain or hill is mapped in the middle of an area and not surrounded by water. Zero meters of elevation represents sea level and if it includes 0 meters than that means that land is possibly an island. In other cases, there should be a contour line marked zero, but in some cases there are not.