Optional objective: Limit ally health loss - 75%
Sail carefully between the rocks and move closer to the Independence. You will eventually receive a task of protecting the allied ship from the attacking British units.
Stick close to the ship and wait for the enemies to approach you. The first cannon salvo should let you destroy at least one of them. Afterwards sail around the Independence while shooting all the enemies you pass by.
The protected ship should receive some damage, but it should manage to break out and destroy a few enemies. Follow it while eliminating the chasing enemies. If you're accurate, you should be able to blow up all of the enemies before the allied ship loses 75% of its endurance and therefore get full synchronization.