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.
The "miracle on the Hudson" was an incredible landing. Water landings pretty much never go that well. The usual result is the drag on the landing gear causes the nose to dig into the water or tears a wing off, and then the passengers (if they survive that) have mere moments to get out before it sinks.
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.