Let the water depth h be the water depth of the original position (the part where the flower stem is not exposed from the water surface) corresponding to the short right-angled side of the right-angled triangle, and the displacement of 2 feet is equivalent to the long right-angled side of the right-angled triangle (the flower stem becomes the hypotenuse after displacement, which is h+0.5 feet).

So it is actually a right triangle with two right-angled sides H and 2 and the hypotenuse h+0.5.

According to Pythagorean Theorem, the square of H+4 = the square of (H+0.5).

H=3.75, that is, the water depth is 3.75 feet.