In a saucepan, warm the water with the gelatin to dissolve it. Whisk in the lime juice and simple syrup. Taste for sweetness, and adjust if needed. Stir in the raspberries and pour into a loaf pan 8 1/2 by 4 1/2. Chill at least 4 hours. Dip in hot water for 10 seconds to loosen, then flip terrine out onto a serving platter. Keep chilled until ready to serve.