Can you elaborate on how you pull this off, Windsor? For instance, how do you make sure that you even detect the opponent's fleet?
Once you reach optics you crank out a couple of stacks of caravels as initial scouts, add a few spies to map his territory. When guarding cities, always make sure that you position yourself so that the enemy can't attack and then retreat into the city on the same turn. If he sinks one of your ships, you need to take one back. Then it's really just a matter of spreading out and monitoring his build. This is perfectly viable with even tech, but if you're behind you're in trouble. Naval warfare in Civ4 is very binary. Letting someone slip past once you got the ships in place is sloppy, but I can see gamemode being very relevant here. There's a huge difference playing a simultaneous turn game with blazing timers vs playing a pitboss/PBEM game. I don't have any meaningful experience with simultaneous turns.
And also, this ONLY works 1v1. If you're against more enemies, having all your ships at one enemy will obviously just make the others attack you. If that's the case you might consider settling more cities inland. When settling in MP security is always much more important than in SP.