You've found three message so far but there are still more to find. You can get this mission after accepting the mission "Crazy About You" and it is good to do it alongside this mission
True Vault Hunter
To get to this bottle, you need to enter the Rustyards as part of the mission "The Hermit". After finding Herbert in the Rustyards, drop down from his house and follow the ridge to the south. You'll enter the Coobie Gorge and be attacked by Spiderants. Go through them and watch for a bridge to the left. Cross the bridge and then go right to find a gate that will lead into the ship. Two anchormen will come out. Kill them and the Raiders that come out behind them. After you've killed them, head into the ship and you'll notice a shack on a platform by itself off to the right. Go into the shack and jump up onto the table. From here, creep over to the window and look out. You will find this bottle wedged between the shack and the hull of the ship.
After getting the message, continue along with the mission "Crazy About You" until you've gotten to the third audio tape. This will put you in the Dreg Scrapyards which is beyond this ship. The X can be kind of hard to find for this one. Once you cross the bridge and enter the Scrapyards, turn right and follow the edge of the cliff. As you do so, look left and you should see some stairs. The X you are looking for is under a set of stairs next to a shipping container. You'll have to crouch to get under and dig it up. Open it up to get your reward.