Was John the Baptist the only one who baptized with water?
Actually, no. The Jews had a long standing practice with water. They fully immersed any proselyte in water to introduce them to Judaism. If you became a Jewish convert, you are baptized in water as part of the ceremonial rituals.
But this was a baptism of conversion. The Essenes in the day of Jesus, a Jewish sect based on purity from what they considered corrupt society also dunked in water. Much of their day was spent in purity and cleansing through baths. They were based in the Qumran community in the wilderness, and you may be familiar with them because they were the ones who were copying the Dead Sea Scrolls.
John the Baptist was quite unique in his application of water baptism. First of all, he also immersed completely and watered baptized. Second, he baptized for a completely different reason. As the forerunner of Christ, he baptized people to prepare them for the repentance that would pave the way for the righteousness of the era of the Messiah.
His water baptism was to fulfill Scripture from the prophets as well as in preparation. His water baptism was then changed to water baptism to confirm an individual became a follower of Christ. This is the way we use water baptism in Christianity today.