Stir in minced garlic, minced ginger, curry paste, and serrano ribs and seeds removed, minced; cook, stirring, for 1 minute. Stir in peeled and cubed (1/2-inch pieces) sweet potatoes and water; bring to a boil. Reduce heat to medium-low and simmer, partially covered, until the sweet potatoes are soft, 10 to 12 minutes.