Answer
660 volts.
Work Step by Step
The energy stored in the capacitor transforms into the heat energy that raises the water temperature.
$$PE=Q$$
$$\frac{1}{2}CV^2=mc\Delta T$$
$$V=\sqrt{\frac{2mc\Delta T }{C}}=\sqrt{\frac{2(2.8kg)(4186J/(kg\cdot ^{\circ}C))74C^{\circ}}{4.0F}}$$
$$\approx 660V$$