ما الذي يجب فعله بعد ترك صلاتي العصر والمغرب بسبب انتظار الماء للاغتسال؟

Delaying the Asr and Maghrib prayers intentionally past their prescribed times, due to waiting for water or bathing, is a grave error and a sin. One must repent from this. A Muslim must learn the rulings of prayer and its times. If water is not available, one must search for it and exert effort to obtain it. If it is impossible to find, then one must perform tayammum (dry ablution) and pray before the time expires. If you have prayed the prayers, then repent to Allah. If you have not prayed them, then make them up immediately along with repentance. A person is considered genuinely without water if they have exerted all possible effort to obtain it and failed.