I assume you mean you are playing Axis Japan, against the Allies? Cause that's the most obvious naval war to get into as Japan.
Regardless of the side you'r on, I can give you some pointers. Naval warfare can be confusing and it will end badly if you don't know what you'r supposed to be doing. I'll split it out in a few pointers, feel free to ask more if something's unclear. I'm no master at naval warfare but I've seen my fair share lately. Sorry if this is a bit of a long piece of text but I've tried to put as much information into it as possible. Feel free to ask on about anything unclear.
-First off, if you select 2 squadrons of ships in the same port you get 2 small arrow icons on the top left of the selection screen. One is 2 arrows pointing at one another, one is 2 arrows under one another. The first one merges the 2 units together into 1 fleet. The second gives you a screen where you can move ships from one group to the other. That is how you re-organise your fleet.
-About convoys. 1800 convoys for Germany is overkill in my opinion, especially at war start. Unless you are going ahistoric and build a fleet that can stand up to the Royal Navy, you might want to cancel all overseas convoys as soon as the war starts. No point in having a hge merchant fleet if you cannot protect it. Convoy escorts will go some way to protect it against small groups of raiding submarines, but are nowhere near enough against a proper fleet and they will just sink too.
That said, a ratio of 18:1 convoys-escorts is quite a bit off too. I'm personally not a huge fan of escorts as I like to put that IC into actual warships, but if I were to put in the effort to place escorts on a convoy and count on them, I would bring that ratio up to at least 4:1, maybe even more.
-All that said, what is very important in actual naval combat is fleet composition and how different ships behave. It's perfectly possible to build a powerful fleet around battleships and win the war with that, even against carrier fleets.
There are 2 types of ships in combat, capital ships and screens.
Capital ships
Aircraft carrier
Escort carrier
Battleship
Battlecruiser
Heavy cruiser
screens
Light cruisers
Destroyers
Other
Submarines
Transports
The first thing you want to do in a fleet is mix capital ships and screens at a ratio of at least 1:1. That means have at least as many escorting units as you have capitals, preferably a few more screens than capitals. That has to do with a few things.
First: you much rather lose some destroyers or light cruisers than a battleship or carrier so they can soak up damage.
Secondly they provide a lot of additional AAA defence, which is paramount when facing enemy aircraft carriers.
Thirdly and most importantly: Average speed in combat. The higher the average speed of your fleet, the higher the chance they can get within firing distance of the enemy fleet, or get out when they want to. Gun-based ships want to close in, carriers want to get away as far as possible because they have aircraft that can strike at much longer range than guns.
Then there is another thing to keep in mind. The game assigns priorities to different ship types, with carriers being the highest and subs the lowest (I believe) Screens use these priorities to decide who to protect. Of the capital ships in the game, the order from most to least imortant appears to be:
Aircraft carrier
Battleship/Superheavy battleship
Battlecruiser
Escort Carrier
Heavy cruiser
Now remember screens prioritise this way too, but these capitals themselves do not. What this means is that if you mix carriers, battleships and destroyers in one fleet, the following will happen.
The carriers stay as far away from the enemy as possible. The battleships will close in with the enemy to use their guns. As the carriers are the highest priority, the destroyers will stay with them, leaving the battleships alone to charge the enemy. A battleship without escorting ships is a dead battleship, just think about the hail of torpedoes coming at her that she cannot dodge.