How? Controlling the sea gives a HUGE advantage. Don't forget many trading cities will probably be located along a shoreline, allowing them to also make trade with nations across the sea. I'm sure they will also allow many units to be carried across the sea, allowing for more strategic movement on the attackers part. Of course, if a city is locate in the middle of nothing but tons of land, then none of this matters.
Also note, depending on how the game mechanics work, the requirement of water might be involved. This of course means many cities will have some sort of water running through, or by them.