I have done the first part, and have gotten;

$\displaystyle \sqrt{5}cos(\theta - 1.11) $

I am stuck on part 'c'. I know the equation looks similar to my answer in part 'a'.

so if I change it to $\displaystyle 15+ \sqrt{5}cos(\frac{\pi t}{12} - 1.11) $

Would this be valid? If so how do I find the maximum depth of the water and at what time this occurs from this expression?

Thank you.