Place pork shoulder and water into slow cooker and sprinkle with garlic salt and pepper. Place the lid on top and set heat to low, letting cook for about 8 hours or until the internal temperature is between 170-180 degrees F. Drain liquid from slow cooker and shred meat using two forks. Toss with barbecue sauce, set aside.
Butter inside of bun and lightly toast in a skillet, if desired. Place shredded pork on bottom bun. Assemble with a spoon of cranberry sauce and coleslaw, then bun top.