This is a continuation of Anne McCaffrey's Dragonriders of Pern series.
Humans weren't the only intelligent life form to travel from Earth to the new planet in the Rukbat system. Dolphins, because of their near/equal intelligence (especially when a "chemical" called menta-synth was used) were also transported. An important reason was because Pern, very Earth-like, had beautiful oceans.
Dolphins were to assist man in exploration and help keep them safe on Pern's oceans. During the early years, dolphins proved extremely important in moving the main settlements from the southern to the northern continent as volcanic and other disruptions threated the lives of the newly arrived earth people.
Readis is the son of an ocean-side "holder" of Pern. By the time Readis appears, the relationship between man and dolphin has been lost -- mostly because the humans involved have died through old age, accident or been killed by "thread". The dolphins maintained the tradition and the most intelligent of them held the position of "Tillek" (a dolphin teacher whose position was named after a human, space navy Capt Tillek, who had been greatly involved with the dolphins in the early years).
Readis and his uncle were rescued by shipfish (dolphins) after being caught in a sudden squall. They suddenly realize that the dolphins are making intelligent sounds. While his uncle had heard of this, he had never believed it, but it turns out that the dolphins do not only talk but also prove highly intelligent.
Using their sonar, they even help the medical experts of the planet since xray and similar equipment has been lost as well as the technical knowledge to create them. Readis, with the help of his uncle and various friends, helps reestablish their partnership to help make Pern a safer and better place to live. The story also discusses various other aspects of the planet, its society and history as well as describes a number of important events in the life of Pern.
This report prepared by John McCoy
Readis (son of Alemi - Menolly's brother) goes fishing with his father. A storm brews and the ship is damaged while the two sailors are thrown overboard where Readis injures his leg. They are miraculously pulled to shore by what Alemi discovers are shipfish (thanks to AIVAS) or dolphins. Readis makes friends with the creatures against his mother's advice (Aramina - remember she can talk to dragons) and discovers why the dolphins were brought to Pern.
This report prepared by Rachael