His nervousness isn't hidden from Daine. She can hear his heart rate picking up a little, and she makes an effort to tow him smoothly through the water. Dolphins can 'see' with sound, like bats, she says, her tone light and conversational. That's how they hunt and talk with one another. Right now, there's nothing nearby but fish. The implication being that if anything else should come along, she'll know straightaway.