Trig application problems.

Hi,

I`m having a lot of trouble solving the following question:

The water at a local beach has an average depth of 1 metre at low tide. The average depth of the water at high tide is 8 m. If one cycle takes 12 hours:

- Determine the equation of this periodic function using cosine as the base function where 0 time is the beginning of high tide.
- What is the depth of the water at 2 am?
- Many people dive into the beach from the nearby dock. If the water must be at least 3 m deep to dive safely, between what daylight hours should people dive?

I am really confused about this problem, and dont even know how to start

Any helpful tips will be greatly appreciated.

Thanks! :)