Pour coconut milk and water in a large saucepan. Add curry paste, soy sauce, lime, peanut butter, and cayenne. Gently stir until evenly combined. Adjust spice level by adding more cayenne or curry paste as needed. Allow mixture to come to a simmer on medium-high heat, then simmer on low for 10-15 minutes. Feel free to add more liquid if the sauce gets too thick.