Answer
$\theta = 13.4^{\circ}$
Work Step by Step
In part (a), we found that $\Delta t = \frac{D~sin~\theta}{v}$
In part (b), we found that underwater $\Delta t = \frac{D}{v_w}$
We can find $\theta$:
$\frac{D~sin~\theta}{v} = \frac{D}{v_w}$
$sin~\theta = \frac{v}{v_w}$
$\theta = sin^{-1}~(\frac{v}{v_w})$
$\theta = sin^{-1}~(\frac{343~m/s}{1482~m/s})$
$\theta = 13.4^{\circ}$