If you play a game of rock, paper, scissors, and you know ahead of time what your opponent is going to play,what will happen?
Same thing about a fleet. A carrier heavy fleet is good against for defense against air attack, but is vulnerable to a mostly sub fleet with a few carriers for air defense. A battleship heavy fleet is good for hitting and running, but isn’t good for defense. You almost never need a fleet for “general purpose” because you can see the opposing forces in the area ahead of time, figure on what reinforcements they will build, and plan accordingly. This whole thing about “free” transports is rather unnerving . . .
You should have at least 1 destroyer in the force in case of an enemy sub attack, or the possibility the enemy will bring subs. If you don’t have any destroyers, and are far from reinforcement, the only thing you can do against a massive sub build is going to be to run. You should ONLY have one destroyer per fleet; only build two destroyers if you are planning on splitting your fleet.
(edit) WHY only 1 destroyer? Because 12 IPC gets you 3 attack or defense; you pay 4 IPC per. If you spend only 8 IPC, you get 2 attack or defense, still 4 IPC per, but now you only have to pay 8 IPC per casualty instead of 12. Destroyers are useful to bulk up against air attack, but then, that’s what carriers are for. (/edit)
You should have at least 1 carrier in the force in case of an enemy air buildup. You should also have 3-4 fighters per carrier nearby, so you can replace killed fighters. You can possibly dispense with this and get destroyers instead if the enemy is low on resources and out of air; destroyers can have Combined Bombardment researched and be used for attacks on coastal territories.
You should have at least 1 transport to increase the possibilities of your fleet’s attack.
You should have at least 1 sub for battle fodder. (It is the cheapest naval unit with an attack).
After that, the composition of your fleet will be highly dependent on what you want to do with it. If you want to attack an enemy navy quickly, you need lots of subs, if you want to pose a serious threat on land territories, you need 2-8+ transports, if you want to do some light harassment and hit and runs, you need battleships.