Place Bratwurst, Beer, Onion, And Ketchup In A Slow Cooker. Pour Water Over Mixture Until Everything Is Covered. Set Slow Cooker To High And Cook For 4 Hours. Preheat An Outdoor Grill For High Heat And Lightly Oil Grate. Grill Bratwurst On Preheated Grill Until They Are Browned, About 5 Minutes.