Simple trigonometry.
Imagine a triangle, 30⁰ at the boat, with the hypotenuse being 600ft, and the adjacent being parallel to the water; we need to work out the height, or the opposite.
We use sin(), which is opposite over hypotenuse, as we know the hypotenuse and we want to find out the opposite.
We do
600 * sin(30) = 300ft