Cas Bonner is a marine biologist.
He is possibly one of the very best in the world but since he has a habit of telling people what he really thinks and does not act with the expected reverence for the more established scientific community, he is often not in the list of people to ask for opinions. When he does give his thoughts, smart people tend to listen very carefully but they would likely not want to admit it.
At forty years of age when the four-book series begins, he has just completed three years of imposed exile from the scientific community due to his penchant for speaking his mind and irking the people in charge of grant money. Without the grants, the research does not get done and universities and foundations have no use for brilliant and experienced unfinanced scientists. Perhaps, though, another reason for his ostracism was his annoying habit of being right.
While this description might give the impression that Bonner is a jerk, the reverse is the truth. He is one of the good guys and many people who have worked with him or have known him outside of work would be happy to list Bonner as one of their friends. Bonner is true to his word and loyal to his friends and always there when someone needs him. It is just that if a person is expecting a more respectful demeanor when that person has not shown any reason to deserve it, there is going to be disappointment.
In the field, especially deep beneath the ocean surface, Bonner is considered daring without being thought foolhardy. The ocean is his depth and he knows and respects it greatly. He realizes constantly that the sea does not care one iota for anyone and death will come quickly to anyone who does not respect her. Nature is one institution to whom Bonner shows the greatest respect.
Bonner is in this collection of spy characters because during the period of his exile, during which he earned a living running a charter fishing boat out of a marina in Honolulu, he responded to a plea for help from an old friend, a response which got him involved in a hunt for a sunken cargo ship with a nuclear payload onboard. That adventure brought him to the attention of the intelligence and military communities and when other serious matters came up which required his expertise, he became the man to call.