From Ripleys –
Marine mammals are also used to patrol ports and act as lookouts for ships. With their keen senses, they can spot enemies and even be trained to attach recovery lines, tracking devices, and even explosives as needed. Dolphin and sea lion sentries were first deployed during the Vietnam War, and then later in Bahrain during the 1980s.
In addition to all of that, dolphins are often used to assist human divers who become lost. The Navy has also trained them to recover bodies from sunken planes and lost munitions when the threat to human divers is too high.