Combine vegetable oil, garlic, red pepper flakes, and ginger in a large skillet over medium heat; cook and stir until fragrant, about 1 minute. Add sugar snap peas; cook until bright green, 1 to 2 minutes.
Stir water and cornstarch together in a small bowl until smooth; pour into the skillet. Add shrimp, coconut milk, and tomatoes. Increase heat to medium-high; cook until coconut milk is simmering and shrimp are light pink and firm, about 7 minutes. Sprinkle Thai basil on top.