Pure speculation here, but if the aircraft had broken up in-flight at its cruising altitude would not wreckage be spread over a very much larger area and consequently be more likely to be spotted quickly, compared to the concentrated wreckage field which would result if the aircraft had impacted the sea more-or-less intact?
Yes, spread over larger - no, for spotted more quickly
If the debris is all together the visibility would by better from the air as it would be a large dark anomalie in water.
To find a single part, as large as a rubber boat, in an area as large as, flying at probably 7000 feet or higher, is almost impossible.
Apart from this, think on all the rubbish cargo ships tend to loose all over the place. This reminds me on a movie i was just watching, "all is lost" with Robert Redford. Unless you put a big light on, no one will find you.