1. Make a BSpline Surface from the Create menu (Create -> BSpline Surface...). Create a fourth order surface 10m long, 3m for the half breadth and 2.5m depth.
This will create a flat surface 10 units long in the x direction, 3 units long in the y direction and 2.5 units in the z direction. The surface control points will have four rows and four columns.
2. Select the surface and edit (Right-Click ->BSpline Surface -> Edit) or press F2. Change to view to look at a sectional view of the surface (Right-Click -> View -> Stations). Move the inner two control points to form the curve of a hull section. As the surface is displayed in an orthogonal view and the control points on the row have the same y-z coordinate, the entire row of control point can be edited at once.
Refining the Surface using Subdivision >>>