The depth of water in a harbour is h = 6 + 2.5 sin (π t)/6 metres, t hours after midnight…

Step 1. The extremes. The greatest depth is $6 + 2.5 = 8.5\ \text{m}$, and the least is $6 - 2.5 = 3.5\ \text{m}$.

Step 2. The period. $\dfrac{2\pi}{\pi/6} = 12$, so the high tides are $12\ \text{h}$ apart.

Step 3. The first high tide. The sine is $1$ when $\dfrac{\pi t}{6} = \dfrac{\pi}{2}$, that is at $t = 3\ \text{h}$, at 3 in the morning.