Is it obligatory to deliver water beneath the peeling and raised skin due to a skin disease in the feet, especially what is located beneath the toes and is difficult to reach?

If the skin is elevated but not cracked, water should only reach the surface of the skin. However, if it has peeled and cracked, water must reach the inside of the cracks if possible without harm. If you doubt that water has reached the skin in the cracks during ghusl (definition), you must repeat the ghusl.