In The Time of Dinosaurs

Ax was in his disturbingly attractive human morph. As opposed to his disturbingly, disturbing true Andalite body. He had to return to his own form before he could go into another morph. He'd picked up his newest morph at The Gardens - with Cassie's help, of course.

—M2C1

[...]

Rachel was on my right. Ax on my left. We advanced across the ocean floor, blasting the water with our ultrasonic sound waves. It had taken forty-five minutes of hard swimming to reach the site. We couldn't go beyond two hours in morph. Not unless we wanted to spend the rest of our lives as dolphins.

—M2C2