Optional objective: Complete the contract with minimal allied casualties - 4.
After reaching open sea you will receive a task of protecting the merchant ships from four British ships. To complete it with full synchronization, try to shoot very accurately and keep an eye on the sea around you so that you don't lose a single chance to shoot an enemy.
You should sink 1-2 ships before they all split up and start sailing in opposite directions. Enemies aren't very accurate, but if you leave a ship for too long, it will surely get destroyed.
Destroy the last ships and the mission will end.