Every weapon has their strength(s) and weakness. If you use your weapon's strengths, you can determine a battle's outcome.
Cannon type weapons.
These weapons include 20.3 cm and larger cannons.
When using these weapons, it depends on what kind of ship you're fighting. Most of the time, you won't be fighting another ship of the same class. But in general, these cannons are brawling weapons. At long range (About 2/3 of their max range), these cannons are inaccurate, and take time to get to their target. This makes these weapons easy to dodge at longer ranges. But when facing these weapons at shorter ranges, they can heavily damage lesser vessels.
The best way to use these is to aim in front of the enemy ship, especially if they're a fast small boat. At shorter ranges, you can and will generally hit them just by aiming at their ship.
These weapons are the 12.7 cm gun (Both the American and Japanese versions, not the Mk-45 127 mm), 15 cm gun, 12 cm gun, 47 mm and 76 mm (Not the Oto Melara 76 mm) guns and Vulcan cannons.
These weapons are all cheap (Except the Vulcan cannon which cost crystals), fast firing (Compared to the larger cannons), short range (The highest being the 15 cm gun), low damage, small size. All of these weapons are mainly secondaries and or a cheap manual AA.
These weapons are and can be used by every ship in the game class due to their cheap cost and small size.
Flat trajectory cannons.
The two current weapons are the Mk-45 127 mm cannon (Mk-45 5 inch cannon) and Oto Melara 76mm gun.
These weapons both have a low firing arc and are both very accurate, which allows these weapons to have excellent low flying AA potential. They also serve as a small but powerful weapon, with the 76 mm taking a 1x1 space. While the 127 mm takes 1x2 space.
These weapons are mainly harassing weapons, or weapons to snipe boilers and cripple the enemy's speed.
There are five current torpedos, those are, 450 mm Torpedo tube, 533 mm Torpedo tube, 610 mm Torpedo tube triple, Underwater Torpedo tube, Oxygen Torpedo tube.
Most of these weapons have short range (Except the Oxygen Torpedo Tube), are inaccurate (True for all torpedos), and are at a fixed position (Except the Oxygen Torpedo Tube which has full turret rotation). The most powerful of the group would be the Oxygen Torpedo Tube which has a 22 km range and the highest damage potential (An average of 4875 in fire power with 4 torpedos).
All of these weapons are deadly if used right, as you need to close into almost point blank for a full salvo of torpedos to hit. Of course, with once again the Oxygen Torpedo can be played differently, with the most common being a torpedo shotgun. This involves using tens to near hundreds of Oxygen Torpedos which makes a 22 km no man's land.
There are currently only four weapons that can hit a submerged submarine. These four weapons are Underwater Torpedo tube, Depth Charge, Hedgehog, and Mines.
These weapons can be used on other ship types besides submarines, but are the only ones that can destroy submerged submarine. All of these weapons are very close range weapons, and also deal a lot of damage if they hit their target.
There really isn't a strategy if using these weapons. These weapons are mainly just for defeating one ship type (All except Underwater Torpedo and Mines).
Any ship in the game can have these weapons.
The two weapons that contain missiles are the VLS (Tomahawk) and the RGM Harpoons.
Both of these weapons have limited use (VLS having max being 16), have an extreme range (VLS is 88 km and Harpoon is 62), are easy to dodge (If in a small and fast ship), can be taken out by CIWS. Both of these weapons also cost a lot. Due to their high range they are the best weapons for grinding airbases, if you have enough.
The best way to use these weapons (the tester has found) is to cripple their speed so they slow down. After they've slowed down shoot all of your missiles. Of course, this is my (the tester's) strategy, you can do what you want with your ship and explore whatever in-game factors you please with your ship.
These are mainly used on certain Fast Boats (The missile boat subclass), some Destroyers, some Cruisers and Battleships, and some Submarines. Missiles can be used by any ship in the game, and their accuracy can be increased by equipping the AN/SPH-62.
Anti-Air Guns (And rockets) and CIWS.
These weapons are the 7.7 mm Gun, 37 mm Gun, 57 mm Gun, 40 mm Bofors, 25 mm Triple Type-96, and also the 120mm 28 Rockets. There are also two CIWS but CIWS will include both of them as they both do the same thing as each other.
All of these weapons are upwards firing AA guns. Meaning they will try to shoot down dive bombers, and torpedo bombers after the torpedos have been dropped. Disclaimer, AA guns will NOT shoot down low flying torpedo bombers. After the torpedo bombers dropped their payload, they rise up and then come into the AA gun's line of fire. This also applies to fighters (If they are not overhead). But this does mean that they can and will shoot down attacking dive bombers due to the fact they attack from above. Rockets, on the other hand, are a slow firing spray shot AA weapon. Rockets can and will hit low flying planes, but some will still get by due to the rockets slow reload.
All of these weapons have a very good fire rate (About one shot a second), have enough damage to down a plane in one hit (If the 7.7 mm hits, the plane will slowly descend), low range (The 57 mm has 10 km range, but can't hit anything till they are around 2 km), small (All are 1 by 1), are all angled upwards, have flak rounds (Flak rounds are shots that explode, every AA gun has these but the 7.7 mm Gun). As for the CIWS, they will target missiles and the bombs dive bombers drop.
AA guns should be put in batteries, like a huge clump. Yes, this means one hit could destroy all of them, but this creates a huge death zone above that area. AA guns should not be relied on for facing higher tier carriers due to torpedo bombers. In that case you should combine AA guns with the Mk-45 5" gun for a carrier's nightmare. CIWS can effectively take out any missiles headed for you, and also render a carrier's dive bomber useless.
These weapons can be used by every ship, but submarines (You can put them on it, but it won't fire underwater). Also, you don't need to have these, as they are optional. Keep in mind both of the CIWS cost crystals, but will very effectively destroy any incoming missiles, even if they have the AN/SPH-67 equipped.
Most would agree planes need their own category in the weapon strategies, why you ask? Planes are special because they control themselves through whatever AI scripts the devs of this game coded them with. Therefore the only thing you can do as a player is launch them, they land on their own (Sorry for pointing out the Obvious), to cover planes we will factor in both types, fighter & bomber, keep in mind bombers have sub categories by the names of dive bomber & torpedo bomber.
Fighters are mainly used to counter enemy bombers and keep enemy fighters away from your precious ship. It is suggested to keep fighters in the front of your plane lineup on your ship so they get out first and can much more easily protect your bombers. To make fighters launch first, place the fighters right next to or as near as you can to the runway.
After your fighters are deployed your bombers will follow. it is highly suggested that you always have fighters with bombers because they can allow the bombers to have a higher survival rate if the enemy has fighters, therefore do more damage. Torpedo bombers stay low to the water and dive bombers stay high above the water, torpedo bombers get close to their target & deploy a torpedo then flee back to your ship, meanwhile once dive bombers get close they dive down and deploy bombs on the enemy ship then flee. It is all up to you how you use any of these planes :) .