"Use one broadside to sink two boats" is an optional mission objective in Sequence 3 / Memory 06: Proper Defenses
Walkthrough[edit]
After weakening the enemy ships some, sail into close range, align yourself with a group of two or more enemy ships, and fire at them with your Broadside Cannons to sink two boats at once. Heavy shot (broadside fire without aiming) works very well in dealing enough damage to sink two weakened boats in a single shot, and counts towards the objective.
