d(t) = 3/2 sin (5πt/31) + 23
The average depth of the water, over time, is 23 feet.
The amplitue of the tide is 1.5 feet
So the maximum depth of the water is 24.5m
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You do not need this but the period is 2pi divided by 5pi/31 = 2pi*31/5pi = 62/5 = 12.4
I think it fair to assume that we are talking tides and time is in hours.