If no one got in or out of the boat for watersports or swimming then that would indicate you're taking on water. Is the boat an inboard/sterndrive or an outboard.? You could have a leak in the drive belows if an inboard/sterndrive, or a leak in the transom or splashwell if an outboard. Check the hull for any damage. Also inspect your transom closely. I believe the Starcraft American is a fiberglass run about correct? If it's an aluminum Starcraft that would present a whole different group of items to check for leaks.