Heat butter in a microwave-safe measuring cup in the microwave for 30-60 seconds until melted. Stir in beer, shrimp boil, salt, and garlic powder. Microwave 30 seconds longer to melt any butter that solidified in the cold beer.
Use a flavor injector to inject the beer marinade into the turkey breast. I injected the needle into the meat every inch or so. Be careful because some of the liquid can shoot back out, so it can be a little messy. Inject into both sides of the turkey breast until you’ve used it all.
Place turkey breast-side down in the slow cooker, and cook on low 3-4 hours until internal temperature reaches 165 degrees F. (This is best measured with a probe thermometer. The doneness indicator did not pop up on my turkey in the slow cooker.)
Remove turkey from slow cooker and allow to stand for 10-15 minutes before slicing.