You state semi-elliptical in the heading, but circular in the problem. It is

an ellipse with semi-major axis length 15 and semi-minor axis length 10.

The equation of said ellipse is

The area of the rectangle, designating the boat, has area A=2xy

Solve the ellipse equation for y and sub into the area formula.

Differentiate w.r.t. x, set to 0 and solve for x. y will follow.