A Deadly State of Mind - S4-E6
Other mistake: Right before Columbo tricks Doctor Collier into incriminating himself, he places Collier under arrest, even showing him the warrant. At no time does Columbo inform Collier of his Miranda Rights; meaning that Collier's self-incriminating statements would not be admissible in court. Although the series typically features Columbo cornering the culprit in a way that is unlikely to hold up in court; this particular example stands out as Columbo has torpedoed his own case. If he had not placed Collier under arrest and let him incriminate himself then the statements (in front of witnesses) would be admissible as evidence.
Other mistake: When Rosanna's body is brought to the ship's medical room, the doctor confirms that there are no bruises on the body, but she had been forcibly slapped in the face earlier that day by Mr Danzinger (Robert Vaughn). The slap was hard enough to knock her over therefore a bruise should be visible, even through make-up.
Answer: "Apparent" drowning answers your question - things are not always as they seem. Drowning could be accidental, but it could also be a murder in disguise. Moreover, the actual cause of death has not yet been determined - accident, suicide, murder, or natural cause (e.g, heart attack while swimming). Columbo would be there to investigate if anything looks unusual for it to be a mere drowning or if there is evidence or suspicion of something else.
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This was just on TMZ.com's "Aaron Carter Dead at 34" (11/05/2022): "Law enforcement sources tell TMZ... homicide detectives have been dispatched to the scene but we have no information or evidence of foul play. It's standard operating procedure for homicide detectives to investigate such [drowning] death scenes."
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