I would begin by drawing a line segment from the center of the semi-circle to the edge of the water, and a vertical line from the center of the semi-circle to the water. You now have a right triangle whose legs are w/2 and 3 - h (where h is the depth of the water), and the hypotenuse is 3. Use the Pythagorean theorem and solve for w.

Once you have this, which is w(h), you want to express h as a function of t. Can you proceed now?