Age of Darkness is a mod set in the historical period spanning the two world wars of the 20th century, revolving around the struggle between the forces of liberty, socialism and reaction.
Although naval units don't often get upgrades in Age of Darkness, that does not mean that naval combat is completely neglected. On the other hand, naval units have had their costs bumped up, but their armour, hitpoints, range and attack have also been increased in tandem, and destroyer and battleship type units have added attack against buildings.
For Age of Darkness, you can create cheaper vintage weapons from the anchorage (namely the destroyer, submarine and cruiser) or more powerful but costlier weapons (including the carrier, submarine, battle cruiser, and battleship).
There however is another class of units - fast attack craft. Available only to German, Italy and the USA at different points in time, these units are highly mobile and while rather vulnerable to destroyers and aircraft (not to mention land batteries) they are useful especially for being cheap and (generally) faster than all other naval units. Among the three, the PT Boat (the others being the MAS and the S-boat) has middling offensive and defensive parameters and cost-effectiveness, but has the benefit of having the highest speed and a propensity for destroying any and all land units it encounters, but is useful only for attacking enemy aircraft and is useless against submarines.
In all cases, each unit consists of 2 warships.