Determine a sinusoidal equation.

A boat tied up at a dock bobs up and down with passing waves. The vertical distance between its high point and its low point is 1.8m and the cycle is repeated every 4 seconds.

a) Determine a sinusoidal equation to model the vertical position, in metres, of the boat versus the time, in seconds.

b) Use your model to determine when, during each cycle, the boat is 0.5m above its mean position. Round your answers to the nearest hundredth of a second.

I have a few questions..:

The answer for a) is y=0.9sin(pi/2t) why does that equation not show metres.. if I were to have my answer as 0.9**m**sin(pi/2t) is that wrong? Also why is it sin? Why can't it be cos or tan? For question b can someone give me a hint as to what to do (Thinking)