Hmm, currents are an interesting idea. I would prefer some sort of graphic to indicate currents, maybe a slightly lighter blue and some animated waves going in the current flow direction (I am hoping for animated water in civ4, that water even in the early screenshots looks really nice).
Some things to keep in mind: Currents early on would be a big exploit for the player, who would be sending triremes down every current showing, and beating the AI to the other land all the time, so currents maybe shouldn't even show until you discover an Ocean Faring tech later on, right before Magnetism.
Dead zones with no wind would be far more realistic for becalming ships, I don't know how you'd represent that though.
In the spirit of animated water, I'd like to see some animated storms, too. These could be frequent in the ancient age, and force galleys to put in to shore constantly. If you take a galley out to sea, you will probably see storms at every turn, and perhaps have a chance to run from them if you haven't used all your moves yet. You would probably be forced in a direction you didn't want to go, though. Stop in a storm and you sink. Your oarsmen should get tired after awhile too, as you would run out of food, so move points for galleys should be reduced after a few turns at sea, that might put an end to the suicide galley strategy.