Haradrim, you reached a point. What probably is the less realistic thing in Civ III is that there could be unused tales. Not only sea ones, but land ones. As you all said, there are sea tales with ressources that would never be exploited by any city. Just like we should be able to build farms, mines or scientific buildings in non-city-influence tales, we should be able to create fishing itineraries and oil rigs.
Those can be linked to whatever city you want, of course not too far from the tale. This linakge should be changeable on every turn. You could it with ones that are within your cultural borders, without generating costs; for the ones outside of your borders (and of course not in another civ's borders), there would be costs.