Return to camp and speak to Reverend Swanson to trigger the beginning of this mission. Dutch wants to help Rain Falls retrieve his horses from the army and Arthur tags along with Charles to keep him in line.
Once you reach the marked area, dismount from your horse and get onto the canoe. Row up to the side of the boat and climb onto the deck, you’ll see the first soldier in front of you, head behind him and perform a Stealth Takedown. Head down the right side and wait for the next soldier to walk out and face away from you, head behind him and perform another Stealth Takedown while Charles takes out the other. This fulfills the requirements for the Gold Medal, walk a little further forward and you’ll spot three more soldiers playing poker. Enter Dead Eye when they come into view and put headshot all three of them to clear the boat of any resistance. All that’s left to do now is to put dynamite on the anchor (Arthur won’t make the best sailor that’s for sure).
Once the boat has begun sinking and the horses are set free, quickly swim over to one of the three horses on the left and mount it. Now gallop as fast as possible back to the shore, the other two horses will follow behind so you don’t need to worry about it. You should have more than enough time to reach the shore within the 1 minute 30 seconds required. Once you reach shore, dismount and head over to Dutch, when given the choice to help Rain Falls, choose to help to unlock two missable Story Missions.