Actually, this game could use a battlecruiser, and I agree that PT boats would be a little difficult to implement, but with cheap air units ruling the high seas because of fewer sea zones than what are needed, a cheaper naval fodder unit is needed.
Actually, I would like to see the entire naval unit structure to be altered.
I haven’t spent a lot of time on this list, but maybe something like this would work.
submarine 2 attack, 1 defense, 2 move, 6 cost - special abilities - same as OOB
destroyer 1 attack, 1 defense, 2 move, 5 cost - special abilities - same as OOB
cruiser 2 attack, 2 defense, 2 move, 9 cost - (gets 1 aa shot at attacking air units)
battlecruiser 3 attack, 3 defense, 2 move, 12 cost - can shore bombard
pocket battleship 4 attack, 4 defense, 2 move, 15 cost - can shore bombard
battleship 4 attack, 4 defense, 2 move, 19 cost - can shore bombard, takes 2 hits to destroy
escort carrier 0 attack, 1 defense, 2 move, 7 cost - may hold 1 fighter
aircraft carrier 1 attack, 2 defense, 2 move, 13 cost - may hold up to 2 fighters