It's been a long time since I bred and owned aquatic turtles and personally, I never encountered this issue.

That being said, I knew someone who ran a large scale turtle rescue. She mentioned dehydrated turtles being unable to submerge themselves. I can't remember the treatment she suggested (other than re-hydration). There was something about adding electrolytes to the water, but I would seriously consider consulting a veterinarian.

I hope this helps in some small way, and I wish I could do more to help. Good luck!