As with Empire, I see a lot of posts about people ignoring the naval conflict aspect of the game and just using auto resolve. IMHO this is a shame as the naval warfare is the best part of the game. The purpose of this tutorial is to give a very basic explanation of how to beat an equivalent AI fleet on normal difficulty with results better than auto resolve (and more fun too), without needing to micromanage a fleet to death. Remember also that these are basic tactics. If you want to take down a smart human player or kill a SOL fleet with three frigates you'll almost certainly have to do something more complex.
1. Fleet Composition.
Unlike land warfare, with its varied terrain and different unit types, naval warfare is generally on an open ocean with various sizes of floating artillery batteries. Therefore, to take down a standard AI fleet the composition is pretty straight forward - more ships than the enemy, and heavier ships than the enemy. Generally, the toughest AI opponent you'll face is a late-game British fleet, which will typically consist of four 1st-2nd rates, and the rest 3rd or 4th rates. To take down a fleet like this I like to use three 122 gun ships, and the rest 74 or 80 gun ships. Your 74 or 80 gun ships are the best balance of firepower and sailing qualities, but lack the sheer firepower and armor of heavier ships. 1st and 2nd rates, by contrast, have the armor and firepower but are hard to maneuver. Generally, I think that you want at least half of your fleet to be 3rd rates for two reasons: (1) you don't need more than a few heavy ships to take down an AI fleet; and (2) the extra speed of 3rd rates is helpful once the enemy routes and you need to chase down ships. Otherwise naval battles could take a long time as your fat 122 gun ships try to chase down something.
tl;dr version. Build 3 122 gun ships and 7 80 gun ships. If you have a crap naval power like Russia or Austria, build 10 of your best ships (80s or 74s). If you have England, just add 80s to Nelson's starting fleet.
2. Tactics
Since the terrain of water is basically flat, and your only real environmental factor is the wind, the basic strategy is to isolate part of the enemy's fleet and kill it with superior firepower. What we're going to do is isolate the van of the enemy fleet (i.e. the lead ships) and kill it before the rest of the fleet can help. Since the AI puts it heaviest ships and Admiral in the van, this will cripple them beyond their ability to recover.
A. Set-Up
To keep things simple we'll arrange our entire fleet in line astern formation, using round shot, and keeping fire at will on (the default settings). This will allow us to control the entire fleet without micromanaging. We'll use tactics instead of micromanagement to line up our shots on the enemy. The only tricky part here is that if you just click on the line astern formation, the computer will auto-arrange your ships how it wants to, always placing your admiral in the first ship. Ideally, you want your admiral in the third ship. The first two ships tend to take the hits, and the enemy is reloading when the third ship passes, keeping your admiral safe (FYI, this is why I like three 122 gun ships, the first two to tank, the third with the Admiral). So line up the ships manually, and then group them all as one group. Then press the start battle button. Once the battle has started, you can then click on the line astern formation and the ships will move into formation but keep the order that you placed them in (i.e. heaviest ships up front, admiral in the third ship).
B. Head Straight Towards the Enemy
Since we're keeping it simple, we're just going to go straight at the enemy fleet. The AI will always bite on this because it basically wants to sail parallel to you and just exchange broadsides.
C. Opening Phase
The AI will drift to one direction or the other as it closes, allow it to do so and sail parallel to the enemy fleet until your lead ship reaches just before the midpoint of the enemy fleet (i.e. the fourth ship in a full stack enemy fleet).
D. Breaking the Line
Now you want to turn your line to break the enemy's line. Start your turn just before the midpoint because it takes a little while for your heavy lead ships to make a turn, especially if the wind is against you. If you start turning right after you pass the fourth enemy ship, you should end up passing just after the fifth or sixth enemy ship. As your ships break the line, you should get some nice raking shots.
E. Isolate the Van
Having broken the enemy line with your lead ship, turn your line back towards the lead enemy ship. Basically, you're creating a U-shape around the lead enemy ships allowing all of your ships to shoot at half of their ships. Once you sink or route the heaviest enemy ships (including their admiral), the battle is pretty much won.
F. Clean Up
While you are concentrating on the enemy van, your fleet is also taking shots at the trailing enemy ships as you cross through their line. Thus, by the time you finish with the van, the trailing enemy ships are fairly shot up as well. At this point either just loop your line back towards the enemy rear, or detach individual ships and just line up shots by micromanaging. I know I promised no micromanagement, but by this time things are pretty chaotic and it may be easier to simply line up shots with individual ships rather than trying to maneuver a whole fleet. Also, this is where those third rates will shine as you chase down routing enemy ships.