Even if they wash the car six ways 'til Sunday, they won't get out every spot if there's any blood. But you'd have to use first class forensic tools, like high intensity light sources to pick up blood or any other bodily fluids. But if they buried her at the beach, and then for some reason dug her up and dumped her in the sea, were they getting rid of traces of sand? (If they had her body in the car transporting her to wherever they dumped her in the sea.)
The biggest problem is that there might not be any blood, especially if she were smothered or strangled.