I was curious, so I did a little research on sea turtle yawns:

Lack of Oxygen: Yawning allows turtles to take in more oxygen, similar to how humans yawn. When a turtle yawns, it opens its mouth wide, allowing a significant increase in the amount of air that enters its lungs. This extra oxygen is beneficial for their health, as it helps deliver more oxygen to their bloodstream and expel carbon dioxide.

Hot Weather: Recent research suggests that heat plays a larger role in triggering yawns than a shortage of oxygen. Turtles tend to yawn more frequently in warmer environments and less often in colder ones. So, underwater yawning might help regulate their oxygen levels and other physiological functions like heart rate and respiration.

Stretching and Relaxation: Yawning also serves as a way for turtles to stretch the muscles around their mouths. It’s a sign of relaxation and comfort in their environment. If you want to assist a turtle in breathing, encouraging yawning can be a simple way to do so.

Whatever the reason for the yawn, it's cute! *s*