Key Out of Time by Andre Norton

This is Book 4 in the Time Traders Series, In this book Ross Murdock and Arthur Ashe continue their adventures in Time and Space on the World of Hawaika. Hawaiian and Polynesian settlers help Ross and Ashe discover the way the world has changed from the data tape to present time. Helped by a girl (Karara) and her two trained dolphins (Tino-rau and Taua)