Preheat an outdoor grill for high heat, and lightly oil the grate.
Grill pork tenderloin until seared on each side, 2 to 3 minutes per side. Transfer tenderloin to a slow cooker; add potatoes, green beans, and water.
Cook tenderloin in slow cooker set to Low until pork is slightly pink in the center, about 4 hours. An instant-read thermometer inserted into the center should read at least 145 degrees F (63 degrees C). Season with salt and ground black pepper to taste.