Great discussion topic.
Some thoughts below from my recent implementation of a Global War variant.
As usual, both Argothair and CWO Mark are on-target. In light of similar concerns, I’ve developed and tested a number of new mechanics to make naval warfare more interesting and useful.
First, convoy zones are boxes out at sea rather than in coastal zones that can be easily defended by aircraft. A player that wants to prevent convoy raiding must devote naval units to defending these boxes, which represent merchant sea lanes.
Second, aircraft stats are revised so that each unit has separate attack and defense factors for ground/sea and air targets. Players declare which targets their air units are attacking or defending against before each round of combat. Fighters are medicore ground attackers, whereas tactical bombers excel in that role but have a measley 1 defense against aircraft on a 1d10.
Third, I added significantly more sea zones and adjusted movement capabilities so that the greater relative speed of cruisers and destroyers gives them added versatility over heavier battle wagons.
Fourth, I added both Naval Combat Search and Evasion mechanics. Would-be attackers must first search for enemy vessels in a hostile sea zone before combat occurs. On a failed search, both players occupy the same space but nothing happens that turn. A search can be improved by bringing along air units, especially sea planes. Evasion works in reverse: a player seeking to move through a hostile sea zone without engaging in combat must perform an Evasion, with bonuses granted to submarines.
Naval Combat Searches add a terrific element to gameplay because they impose greater risks on those who choose to attempt combat without imposing too steep a cost (since an attacking fleet would presumably have the advantage over a putative defender in the first place).
Fifth, I added an AA mechanic to cruisers and battleships, which now fire against enemy air units on a paired basis, hitting on a 3 or less. If a cruiser is in company with a capital ship, it can Support, raising both unit’s air defense values to 4.
Sixth, I gave carriers an anti-air defense of 3 but zeroed out their defense against other sea units.
Seventh, I added battlecruisers, which are superior to cruisers but weaker than battleships in terms of defense values (6/5 versus 6/7). Battlecruisers may move 2 spaces, while battlecruisers move 3, cruisers move 3, and destroyers move 4. Fleet carriers move 2, whereas escort carriers move 1.