Hire pirates cannons and blast enemies.
The Pirate Cap is the owner of the smaller ship to the left. For a small sum of money, he can be paid to fire the cannonade battery on the starboard side of his ship:
A second after that, the cannons will fire on the selected spot, dealing 60-120 damage per shot to enemies in a small area. There's no cooldown.
The Cannonade can be used in a similar manner to Rain of Fire, with the limitation being money rather than the cooldown. Its shots can be very efficent against dense groups of enemies or to catch lone leaking enemies, but not so much against tough enemies like Executioners.