Probably not as much as it should have done. I have a little experience in a similar field and whilst all what he claimed was plausible his level of confidence and wording was most certainly not.
You can have the best modelling in the world, an innate understanding of a stretch of water and yet, it will generally catch you by surprise. There are always too many changing unknowns lurking below the surface and even on the river banks etc that mean you are always playing catch up. I've seen recoveries of large vehicles take way longer than expected, simply because they are not where they were expected to be. This is even when the entry point is known and the body of water is somewhat contained rather than fast flowing. Unlike a car, a body doesn't simply sink and stay there.
The diver guy hampered the investigation, sold out the police (sure, they didn't do everything brilliantly, especially communications wise, in this case but the search they carried out was what you would expect). It fed the subsection of 'brilliant minds' that seek out alternative truths and realities.