There are plenty of flat stretches all over the world: in Arizona, Kansas, West Siberia, Mongolia, all over Sahara, Egypt, Sudan, north of Argentina, etc, etc. There's no need for more than several dozen km. If a out-of-fuel Starship can still maneuver with its rudders, it can likely slow down enough for a rough landing.
As of the ocean, given low waves and the airspeed of say 200 km/h, most likely the craft will withstand it if it has withstood the reentry. At least one Starship demonstrated some limited seaworthiness.
Watch the videos of airliners veering off the runway onto flat ground which wrecks the airplane. The landing gear is designed for flat hard runways, not soft bumpy ground.
There aren't many flat stretches of land.
As of the ocean, given low waves and the airspeed of say 200 km/h, most likely the craft will withstand it if it has withstood the reentry. At least one Starship demonstrated some limited seaworthiness.