Is the unavailability of sufficient water for ghusl al-janabah considered a legitimate excuse for not attending Friday prayer?

You should have striven to obtain water for thel ritual bath (ghusl (definition)) from janabah (definition) before the second call to prayer for Jumu'ah (Friday prayer), as that is the time when one is obligated to go to it. If the water was close to you, and you became occupied with ghusl and missed Jumu'ah, then you must perform the ghusl from janabah first, then search for a mosque to pray Jumu'ah in. If you do not find one, then pray Dhuhr as four rak'ahs. However, if you are unable to find water and would pray Dhuhr with tayammum (definition), then perform tayammum and pray Jumu'ah. If your delay in performing ghusl was due to ignorance or forgetfulness, then there is no sin upon you.