i'd suggest reading a few reports of battles. rarely did a commander have exact numbers, more often than not it was approximate numbers, and even then it could take a while to figure out who you were fighting and how many you were fighting against.
A prime example of this is the jena-auerstedt campaign in 1806.
As another point, england didn't know about the outcome of the battle of trafalgar and nelsons death until a few weeks after the conclusion.
Napoleon, in what can only be one measure of his genius, almost always knew the size of the opposing force. Not exactly, of course, but close. In the early days, he used to spend the whole night before a major battle personally scouting the placement of his opponent's forces. In his memoirs, he talks about the battle of Ulm and Austerlitz, and tells in some detail about his plans. He saw that the Austrians had gotten themselves stuck between a river on one side and a range of hills/mountains on the other, and they eventually converged on each other. If France could push her down into this sock-shaped area (Napoleon's term, not mine), and have a force waiting at the choke point, he foresaw that he could decimate the enemy's forces. He claims that he rode all the way around on both sides, checking out the terrain, and got back to his own camp at 4 AM. From there, he told his general staff the lay of the land, and gave his orders. In game terms, this was an outflank maneuver, I suspect. Anyhow, he destroyed over half of the Austrian army. All because he spent time personally finding out what he was up against.
But, this shows up in his high ratings, rather than in knowing strength of corps.
For fleets, it's totally different. Major fleets were always running across each other, criss-crossing the ocean trying to find each other. They would get glimpes from their outlying ships, but no chance to open battle "that time". If I'm not mistaken, this happened the day before Trafalgar, even. Anyhow, I think there's good reason for not having FoW for ships.
The other reason for not having it is because the naval combats are SOOOOO variable. I fought the same battle 5 times last night, just to see what would happen. Here are the losses (the phasing player is first, and that fleets was by far larger than the intercepting player):
2 & 4
4 & 9
4 & 14
9 & 9
6 & 12
They're all over the map (although, still confined to a rather small zone). I think this is a good game balance issue. If we had FOW for fleets, one would never really know what one was up against.
The other thing is the interception rules. If a fleet is going to successfully intercept, it follows that it knows where the enemy is. This being the case, it's obvious that the interceptor knows approximately the size of the enemy fleet. This kind of thing happened a lot.