I think Keredrex makes a good point about adding subs to the mix to take hits as well as the options a carrier+planes give you, not so sure about the Northern German Fleet strategy though :-) If you already own planes, nothing is as cost effective as a carrier with existing planes and 2 infantry provides better land defense than a single plane as you have twice the number of pieces at about 1/2 the cost. It’s really nice to have 4 planes for each carrier, 2 on ship, 2 on land. Try for land combat where the 2 planes on land attack then fly to the fleet, the 2 fleet planes fly out to land combat then land on well land. It works well if you are trading a territory 3 zones from the fleet and defending the adjacent territory, thinks brits and americans attacking german units along the russian border. Nothing I like better than taking out 4 german infantry with 3 allied infantry + 4 fighters and maybe a bomber. On average you only lose 2 infantry, so what if you lose 3, the russians drive a tank in and out to claim the cash.
I think you need a mix for fleet. You need at least 1 destroyer, I like 2 for the reason you might need a ship to act as a blocking unit to cover a mistake on your behalf or strategically to buy you time to move your fleet without fear of enemy fleet attack as it must stop to fight the destroyer. A sub can’t block and a destroyer is the cheapest unit that can. It also means you can send out 1 of 2 destroyers plus air to attack a seazone that contains more than 1 sub, ie trade an 8 IPC destroyer to sink 12 or more IPC of subs and maybe even luck out and take something out when the sitting duck destroyer is attacked. As Keredrex mentioned, its nice to have subs to take hits and they are the most cost effective way of killing enemy ships. Believe it or not, 1 cruiser worth 12 IPC attacks 2 subs that only defend on a 1, the subs win on defense about 60% and have a slight edge on average for IPC inflicting over taken. I highly recommend using a combat simulator and run some various scenarios to see how battles between various fleet configurations works out. Everyone has their favourite, I like http://www.dskelly.com/misc/aa/aasim.html
I nearly lost a game because of a stupid single jap sub that snuck around south america to harass a small combined brit and american fleet headed into the mediterranean. The brits had moved their fleet in but required american support. The americans attacked a lone jap sub with 1 destroyer and 2 planes, I missed completely, he hit, battle ended. Jap sub attacks US fleet of a CC, CV and 2 planes no backup destroyer and no sub that would have taken the enemy sub hit in the first or second battle. I had to take the cruiser off because the loss of carrier even if the cruiser took out the sub meant the brit fleet would not have had enough defense. Just out of luck, I took the CC off the CV hit on the 2 and was able to provide enough defense to the brit fleet to allow it to live. Just remember subs cannot be taken as hits from enemy planes unless the enemy has a destroyer, sometimes a mixed blessing!