This entire mission takes place on the boat. As such have a quick look around. There is an ammo crate or two and a Diary Note [Note 1/1] on the deck.
After a brief conversation and a free ride, things get a little more serious. The shrimps will begin to attack the boat and will pop up on the railings to attack you. You need to shoot them to dislodge them as soon as possible. If you do not they can knock you from the boat or attack you with their tongues to disarm you for a few seconds. If this happens use the on-screen button prompts to deal with the situation.
Continue to fight off the shrimps, hitting the on-screen button prompts until the mission is complete. As you end the mission a Morality Point will be rewarded.