"The crash site is open ocean"
I'd say open sea rather than ocean and not that far from either land or islands. It isn't that deep. I know the area reasonably well and have dived on some of the islands nearby. The big problem is that the whole area is covered in recently laid down mud from all the rivers in N Java - no hard bottom. Probably the best way would be a very high-res magnetometer survey - but the CVR is tiny in proportion to the volume of the area it could be in