Naval warfare is easy to ignore if you're playing a continental power, but for the UK, Japan or the United States it's just as important as the land front. Here's the bare minimum you need to understand.
Fleet Composition
A fleet splits into two roles: capital ships (battleships, carriers) that provide real firepower, and screening ships (destroyers, cruisers) that protect them from torpedoes and submarine attacks. A fleet without enough screens is a fleet that will lose capital ships to carelessness, not to being outnumbered.
Convoys
Convoys carry traded resources and troops by sea. Each route consumes convoys based on distance, and losing them to submarine or air attacks cuts off that supply. For nations that depend on maritime trade or amphibious invasions, protecting convoy routes matters as much as winning any direct naval battle.
The Submarine Threat
Submarines are cheap to build compared to a surface fleet, and can inflict disproportionate damage on convoys and poorly-escorted enemy fleets. A power with little naval industry can use them to deny the sea to a much bigger rival, instead of trying to compete ship for ship.
Common Mistakes
The most common mistake is sending the main fleet out on patrol without enough screens, losing very expensive capital ships to a handful of submarines. The second is completely ignoring convoy protection until trade starts collapsing, and it's already too late to react calmly.