I assume that the height of the water level can be described approximately by a cos-function. (In real life this assumption is not true)
In 24 hours you have 2 high tides and 2 low tides. If one high tide is at 2 am, then there is another at 4 pm and the next at 2 am, and so on...
The maximum height is 2.8 m at 2 am. Screw all these parts together and you'll get the depth of water as a function with respect to time t:
Now you know that d(t) >= 1.5. That means you have to solve the equation for t:
I got: 2 am < t < 3.92 am or 0.08 pm < t < 3.92 am or 0.08 am < t < 2 am
I've attached an image to sho you the graph of the function.