I think you have to seperate out ancient from modern at sea, but I'm not sure where to make the split. The problem is largely because different game features don't scale properly. I always try to conjure up a picture of what it can mean in the way it is set up.
In the ancient period a turn is many years (first 50, then 40, then 25 for a while). So, what does it mean that a "warrior", say, takes several turns to get from Babylon to Ur? Well, I tend to think of this as representing a gradual extension of influence, perhaps migrating groups setting up villages (not visibly of course) and generally interacting with the land. Now when my warrior arrives at the gates of Ur, I immediatelystart to think in terms of the here and now. not even 25 hours never mind 25 years. My brave troops are going to put these chaps in their place. The two views are mutually exclisive. They have to be because the game models in conflict analogous to battles so that it can represent different types of weaponry and tactics etc.
Sorry for the preamble, but I think it helps to keep that idea in mind when you think about sea travel too. If the game is modelling general influence, a single ancient world boat is hardly a drop in the ocean. So think of the arrival of your trireme as representing the establishment of a considerable amount of sea traffic with intermediate bases and all the infrastructure that it implies over a long period (a few turns). for that reason I think the ancient sea craft have their movement about right. however there comes a time when sea travel is a major element and from then on we get closer to a true match with the game clock (sorry, calendar) just as we do with, for example, railroads.
Now I'm not sure when the switch takes place. It could be argued its from about the 14th or 15th centuries when there was certainly a vast amount of sea traffic in the Med. Or perhaps opening up the atlantic a bit later is more significant. Or perhaps it's the arrival of steam-power that made the biggest difference. whichever, in game terms it has to be represented by the development of certain types of ships. There certainly is a case for modern ships to be much faster relative to ancient ones than in the game. So perhaps the speeds should be stretched out over the eras. They fall behind the changes on land, because of the effect of both roads and railroads. but think how practical it is to mobilize vast sea-power nowadays within one or two years. Obviously not quite as potent as the use of railroads, but pretty vast nevertheless.
The only nagging worry I have is how it would affect game play and balance. But that is for developers, not armchair critics like me
cheers
Algie