I think it depends on which specific step fails. The farther into flight it happens, the narrower the area over which the debris is spread. But, I think being over the entire ocean is unlikely, since the trajectory intersects with the planet, and that intersection point would also have to be in the ocean.
Correction: the trajectory only intersects with the planet prior to engine relight testing. After that it's at ~50km [1] (though to be fair, if they make it safely through the relight, all testing so far shows they're likely to make it through most of reentry)