This is the YA book series The Hydronauts by Carl Biemiller. The first book was published in 1970.
From the back of book 1: "The range was not the entire world. But it was more than 80% of it, and all water. The nuclear war had long since melted much of the polar ice caps. Whole sections of continents were long gone. Hive cities had been set up under the surface of the water.
Set in this world of the future, The Hydronauts tells of a team of marine wardens picked by the International Marine Counsel. There is Kim, the leader of the group; Toby, whose ancestors had come from the forgotten country once called Japan. Tuktu, who came from McKinley City in what once was Alaska; and Genright, who had been selected for warden training in the Hive City deep within a plateau of ancient Ethiopia. Carl L. Biemiller tells of a future environment in which men live beneath the sea, can communicate with dolphins, and can preserve human beings in a state of suspended animation for centuries."
And the Hawai'ian / hybrid people are in book 2 where the Hydronauts meet the Otter-People (among others). There is also a third book with four-footed Ocean People.