Seventh Son by Orson Scott Card

SeventhSon(1stEd)
From Wikipedia:

Seventh Son (1987) is an alternate history/fantasy novel by Orson Scott Card. It is the first book in Card’s The Tales of Alvin Maker series and is about Alvin Miller, the Seventh son of a seventh son. Seventh Son won a Locus Award and was nominated for both the Hugo and World Fantasy Awards in 1988. Seventh sons have strong “knacks” (specific magical abilities), and seventh sons of seventh sons are both extraordinarily rare and powerful. In fact, young Alvin appears to be the only one in the world. His abilities make him the target of The Unmaker, who recognizes Alvin’s powers as those of a Maker—only the second ever, and it had been a long time since the first had walked on water and turned water to wine. The Unmaker works largely through water, trying to kill Alvin in his early years, before he can master his abilities.