Does one who has a wet dream during the daytime in Ramadan and cannot find water for a ritual bath break their fast, thereby missing the ritual bath until the next day?

Wet dreams do not affect the validity of the fast, whether they occur at night or during the day. They do not invalidate your fast, even if you do not perform ghusl (definition) until the next day. Your obligation in this regard is to perform ghusl if you wish to do something that is prohibited while in a state of janabah (definition), such as prayer, due to the Almighty's saying: "And if you are in a state of janabah, then purify yourselves." And if you do not find water and fear that the prayer time will expire, then you must perform tayammum (definition) and pray the prayer within its prescribed time.