A Boat can be upgraded into a Ship for five stars, and a Ship can be upgraded into a Battleship for 15 stars once Navigation is researched. (The total cost of upgrading a Boat into a Battleship is 20 stars.) Upgrading a naval unit does not heal it.
Moving an upgraded naval unit onto land causes the unit to permanently lose its upgrade: if that unit is once again moved onto a Port, it will become a Boat.
- Before the "Power" update, which introduced Battleships, ships shot stones, like Catapults, instead of arrows.