All of our perspective is off regarding our expectation for finding a "debris field". If the plane impacted the ocean "mostly intact", their would be significant debris in a location that could be seen in a search from the air at a relatively higher altitude (2K to 5K feet). In this case you would see hundreds of parts and pieces floating that would catch an observer's eye. Now imagine searching an area of ocean the size of Ohio for debris that may only be grouped together with a few pieces of debris here and there (spread out all over). The observers in the air have to fly lower to find it thus can cover less search area per pass. Otherwise, you have boats patrolling in pattern around the area (again over an huge area). They weren't even sure the oil slicks were from the plane or naturally occurring, which does happen more than you would think.
Here is what confuses me. Usually the "black boxes" send a signal ping or something that makes them easier to locate. I'm not sure how effective those are in deep water, however.
A fellow on a radio show I listened to this morning said the black boxes have about a 3-5 mile transmission radius, and will only transmit for about 30-45 days on their battery power. He (nor I) was not sure the depth of ocean the waters in that region. As a comparison, he did mention that they only found the black boxes of the Air France crash after finding some debris. The transmitter had ceased operating long before they located the wreckage. (They got lucky to find it - or maybe more accurately, just kept at it until they did)
Thanks Tenacoius 1. That makes sense. Thanks fo the comments.