If we call the height of the light house x, then the distance from ground level to each of the boats is:
However we know the angel between the boats is 90 degrees since one is NE and one is SE. So we can apply the Pythagorean theorem:
Solve for x, should be the height of the light house.
See this picture: (By the way, in the water view, the bottom line isn't necessarily a full line. Check the top view for a clearer view)