I picked up a copy of this in my grandmother's house, probably 10-20 years ago. I think my mother said it belonged to my aunt Mary, who probably would have read it around the 1970s (high school / college age). The primary characters were a pair of teenage girls who had an older male professor as their guardian. I'm pretty sure the girls had long blonde hair and blue eyes. I don't remember if the girls were feral children raised by dolphins or if they were part of an experiment by the professor, but they were amazing swimmers who could hold their breath for longer than any normal human. Other than that, I just have a few scenes dancing around it my head. First, the professor kept the girls on a boat, presumably so that they could be closer to water. Second, there was a scene where one of the girls put on makeup to try to be normal, and went off on a date. Her sister observes from a hiding place. The date in question sees the sister watching and assumes that this is a sign that they're both interested and he gets more aggressive. She interprets her date's kiss as an attack and goes off of the side of the boat, leading him to think that he killed her (my vague memory is that said date might have been an assistant to the professor or he might have just been a local townsperson).
The girls might have had a psychic connection with each other, but I'm less sure about that. Unfortunately, I read this over a decade ago, and I don't think I even got to finish the book. The front cover had one of the girls on the boat and I want to say that the title talking about them as "dolphin girls". It might have been part of a series.