I watched the BBC program on sunday. It was very interesting, and well done. From the tests they did, it seems pretty conclusive that there isn't likely to be a Loch Ness monster hiding under the water there. The sonars they were using were so advanced that they even found a buoy (about the size of a football/soccer ball), tied down under the water as a test. Despite that, there was no sign of the monster.
It doesn't rule out there being something though. What if the 'monster' was simply in one of the many caves reputed to be around the loch? There are many possible reasons as to why it might not show up. Seems to me that something like this is highly unlikely to stop people believing in nessie, and going there to see it for themselves.