On one hand, it disappoints me that we are still debunking Charles Berlitz's nonsense accounts about the Bermuda Triangle (which includes the Tongue of the Ocean) over 40 years after they started.
On the other hand, debunking the claims from his book Without a Trace is actually harder than debunking his earlier books because by the time the 1977 book came out, people had realised he was just making up stories without evidence, and didn't put in the same effort to show this.
The Skeptics Dictionary explains:
Over the years there have been dozens of articles, books, and television programs promoting the mystery of the Bermuda Triangle. In his study of this material, Larry Kusche found that few did any investigation into the mystery. Rather, they passed on the speculations of their predecessors as if they were passing on the mantle of truth. Of the many uncritical accounts of the mystery of the Bermuda Triangle, perhaps no one has done more to create this myth than Charles Berlitz, who had a bestseller on the subject in 1974. After examining the 400+ page official report of the Navy Board of Investigation of the disappearance of the Navy planes in 1945, Kusche found that the Board wasn't baffled at all by the incident and did not mention alleged radio transmissions cited by Berlitz in his book. According to Kusche, what isn't misinterpreted by Berlitz is fabricated. Kusche writes: "If Berlitz were to report that a boat were red, the chance of it being some other color is almost a certainty." (Berlitz, by the way, did not invent the name; that was done by Vincent Gaddis in "The Deadly Bermuda Triangle," which appeared in the February, 1964, issue of Argosy, a magazine devoted to fiction.)
In short, the mystery of the Bermuda Triangle became a mystery by a kind of communal reinforcement among uncritical authors and a willing mass media to uncritically pass on the speculation that something mysterious is going on in the Atlantic.
Live Science echoes the debunking.
So, we have a proven unreliable narrator, giving an unsubstantiated report, from a person, about an alleged sighting of an unidentified creature.
There is no reason to believe that this is evidence of a plesiosaurus.