Math functions question *help!!*?

The water depth in a harbour is 21m at high tide and 11m at low tide. One cycle is completelyed appx every 12h

a) find an equation for the water depth as a function of the time t hours after low tide

I want to know what it means when it says "after low tide" or "after high tide"(another variation of the question)

My teacher said when its after low tide its going to be cos ... , and after high tide its going to be sin, So um how do i know which one? and if its -cos or -sin, if i start from low tide do i always look at the minimum of the wave and use -cos?