Modern weapons,

This is a great app, but it could use more modern cannons and aircrafts such as:

4.5 in MK 8 Naval Gun (moderate damage; 27km range; high accuracy)

RUM-139 VL-ASROC (anti-submarine missile)

Advanced Gun System (high damage; extreme range 70km; high accuracy)

F-35 Lightning II (anti-ship; high speed; high accuracy; high damage; long range)

Sikorsky SH-60 Seahawk (anti-sub; moderate speed; high accuracy; high damage; long range)

Make most modern weapons cost in-game currency (100,000+ per weapon) and make new, tougher missions that award more gold.
Also, there should be a way to get crystals by voluntarily watching an ad.

Cinematic Greyhound about Warship Craft, v2.1.15