Design a progression and unlockables system that is rewarding but not over the top and too difficult for our small team to implement.
You already have the 3 different ships, 2 of them you have to earn money to purchase. You can build from there. You could have different tiers of ships (In updates or DLC) such as Destroyer (fast, low damage but fast fire rate, low health), Crusier (Med speed, damage, & health), and Battleship (Slow speed, slow firing speed but high damage, and huge health).
You already have the makings for these as the 3 in game seem to follow the same system. Just have tiers for each boat type.