Thank you for the opportunity to help you with your question!
If the water is salt, the environment would be hypertonic. The actual celery stalk is hypotonic in relation to salt water. The water will flow out of the cell to dilute the environment with higher water concentration. Celery stalk becomes limp and rubbery because it loses water.
Please let me know if you need any clarification. I'm always happy to answer your questions.