I consider it a bug: when an enemy blockades area around your port, you can't work water tiles - it's normal, but wen you put your ships in some tile you should be able to work this tile. In BtS you're not. It's causing very strange thing - in case of stand-off, when both players have the same count of ships (because of 10% of terrain defense it's not possible to break through), then both ship armies stands and enemy can blockade your port. Last time I had a map where there was 1-tile wide water passage. I put my ships there, enemy was two tile away from this point and run blockade. This blockade caused that my port situated inside (enemy couldn't reach that port) was unable work any water tiles. Even worse - I lost access to sea resource and my cities had not enough health. It's senseless as enemy was not able to reach my fish resource, this fish was secured by another my ship, still blockades was working as those fish would be unaccessible to me.