Sometimes diving is about what is not really there. It is used to eliminate where someone is not. If someone is reported drowned I have a professional obligation to see if that is the case. That "story" may have been used to cover an abduction, or homicide, thinking law enforcement may not look due to dirty or cold water. It is a search the same as using a K-9, or with a warrant on a house, etc. If I don't locate something, then I know where it isn't and can look at other options.
We had a murder case about 8 years ago. A guy said he beat another to death with a hammer because the guy pulled a gun. He said he threw it in a local waterway and even showed us where. We, pretty much, knew there was no gun but dove for a whole day to elmininate the "doubt" that it could have been there when it went to trial. If a defense attorney would have asked, why didn't you just look, the jury would also wonder as this was a "life in prison" trial. Life is not at all black and white............it is very grey. What ought to be and what is are two different things. What each of us do affects people that are not directly involved such as family.