The Day of the Dolphin (1973)

Dr. Jake Terrell, who has been training a pair of dolphins for many years, has had a breakthrough. He has taught his dolphins to speak and understand English, although they do have a limited vocabulary. When the dolphins are stolen, he discovers they're to be used in an assassination attempt. Now he is in a race to discover who is the target, and where the dolphins are, before the attempt is carried out.