Set your waypoint for the 'G' in the upper-right corner of the map, set up camp closest to where you currently are in the map and travel to the waypoint. You'll re-appear on your horse in the town of Blackwater, where you'll be outside the building with Agent Ross inside. Head towards the yellow cross to start the cut-scene and the mission...
Once you stop, head out of the car towards the shipwreck down below. Follow the agent and then get onto the boat, go behind the wheel at the back and walk up the stairs on your right. Run all the way down to the opposite end of the ship, run up the stairs here and head up to the yellow dot.
Grab the informant and get ready to shoot the two Indians that appear on your right. Walk down towards the stairs and kill anyone else foolish enough to be up on the upper deck. Watch out for the majority of the enemies waiting for you off the boat on the Jetty.
Use the wheel as cover and lock onto them with LT/L2 before firing for added accuracy. Once you've dispatched them all, head back to the car with the agent.
The car will conk out, so switch to your shotgun/rifle and then get near a rock for some cover. You're best to use your Dead Eye here as they're all grouped together, so you'll get maximum impact this way. Use the right-hand rock as you can move around it to keep yourself away from their bullets for longer. Once they're all dead, you'll finish this mission when the cut-scene ends.