In the original game, it was thought (by players I spoke with and myself anyway) that a carrier unit represented a carrier, a couple escort carriers, a few destroyers and a cruiser or two. A battleship represented the same, but with the battleship replacing the carrier in the line up. Submarines represented 3 submarines and transports represented 3 troop-ships. or whatever. Infantry represented mechanized units (Jeeps, Halftracks) and Artillery.
After years and years of play, with the board growing more and more complex and with the prices of things changing with the addition of other things, my assumptions at least, have changed.
Carrier = Carrier, 2 Escort Carriers
Battleship = Battleship
Destroyer = 3 Destroyers
Submarine = 5 Submarines
Cruiser = 2 Cruisers
Infantry = 1 Battalion or Company
Artillery = 1 Artillery Company
Mechanized = Crap on the field (jeeps, Halftracks, Meat Wagons, whatever the trash is the Infantry is using that they might be able to strap a gun too.)
Fighter - Fighter Squadron down from Fighter Wing
Tactical Bomber - Some unknown fighter type, in squadron form
Bomber - Bomber Wing
As for the Carrier having no defense, I still claim they should have AA ability on Round 1, before combat starts. Give them no defense or attack value otherwise is fine with me.