Ah, the old sea region swap trick. ;) Btw, you can do this to the AI (especially catholics and moslems) with a high battle speed ship like Dromons (speed 4).

Say you have two regions (A and B). Place a dromon in each region and each turn, swap their positions. Their high speed means they will easily get away from the slower catholic/moslem ships and there is nothing they can do about it as their fastest ship is only speed 3. This also works against factions with galleys as the AI groups slow ships with their dromons and neutralises their speed for combating this 'technique'.

Yes it is an exploit, frequently used by the AI.