Pros to winning the water:
Better Eco - Fishing is like booming. You are making vills and ships at the same time. So you get more resources faster. As we all know winning the water in classical basically gives you free food while he is scambling for his. Which means you advance faster and have an economic edge, a crucial part of this RTS game.
Control of the Water - A lot of 1700s dont use this effectively, or just dont know. Since you have won the sea it not only means you got all the fish, you have a virtually untouchable navy. What do I mean? Using seige ships and arrow ships to your advantage. Placing arrow ships at edges of the water where the opponents army is gatherin, going mythic and hitting the seige ship upgrade so it will take out buildings at a range of 30. The latter is the most important. Especially on Mediterranean, upgraded seige ships will take out 1-2 of his tcs {if not all) uncontested. This will screwe him on land and its gg.
Cons to winning Water:
Basically the cons of winning water come from the amount of time you let the opponent live on
Less Pop - Yep. From heroic on, farming beats fishing. What does this mean? To get the same food rate he is getting, you have to have more ships, or ships and farmers. Either way the amount of laborers you have on food is more. And then there is the infamous autoqueue. After winning the water some people still make all 80 vills, or there normal amount of dwaves/gatheres. You will find soon that all those extra resourses that you have go to waste as your army is about 20-30 pop smaller then his. YOu can have all the upgrades too, but quantity still beats quality(in this situation).
Slow land transition - Unless this is an island map, the deciding battles will be fought on land.
By putting all your resources and time into the sea, you have left yourself totally vunerable. Enuff so to even lose you the game to a loki or zues in classical.
Okay so enough with the pros and cons, now for specific maps:
Anatolia - I totally avoid fighting the water on this one. All the fishing spots are right by the docks or at least that is how you should build them. Meaning garrisoning a couple of ships will proctect your food for a while. I can basically defend my side with 2 arrow ships and a couple of garrisoned docks. To take out my fishin in classical you are going to have to makes about 20 arrow ships, more if I am fishing a good chunk of both sides. So you just put over 2000 wood and 1000 gold into the sea, while i put 200 and 100. Even after its taken out you are going to have to delete the ships, so its wasted. But the main reason I dont fight the water on Anatolia is because ALL THE GOLD IS IN THE MIDDLE. I spend my wood and resources to take the middle and tower up, fortify, whatever. Sure once you get your land army rolling in you will seem like you have and endless suplly, but once your 2 3000 gold pits run out you are screwed. And because you put most of it into fighting the water your land army wont be that much anyway. I can't even count the number of times I have seen people lose because of this, and only on this map. Remember the middle is more important on Anatolia.
Highland - A faster classical has never been more important. This map is a Zues/Norse favorite. Zues can bolt and screw with you dock builder with his scout and set you back. Then use his hops to take out your docks b4 you even classical. For Norse they just have to bould their fb in the middle and their army is right wehre it needs to be to take out your dock. Well as long as you classical faster fight it, otherwise, there is hunting by the way of water buffalo, which you can probably eat a good chunk of before they get wise, but on this map whoever controls the middle usually wins the game. Or rather whoever wonders 1st. I swear this has to be the leadin map that is won by wonders. The guy that won the water can't push because he doesnt have enuff pop ofr army. the guy that lost the water cant push becuase he cant get past the wall/ships that are clogging the middle. To avoid a long game build some stuff on the other guys side, you will be glad you did.
Mediterranean - Fight the water at all costs. If he Fast heroics, pull back and garrison then heroic yourself. you should have the edge since you were winning the water 1st and have a slightly better eco to pump out counters to his fleet. Its so important to win this one becuase of the seige ship and age advancement advantage. If you win the sea and still lose the game, you are noob =/.
Midgard - Not worth fighting in classical for the same reason as Anatolia. A couple of awrrow ships and a garrisoned dock can protect the water for a while. Its better to fast heroic on this map and try to surprise your opponents on land since you are so close on this one.
On all these maps is it is ALWAYS good to fish, even if you aent going to fight the water. Just decide if you should fight the water based on the situtation, and not just becuase it is a water map.