Looks like the spline is trying to be done on the wrong axis for what you want. Try each of the X, Y and Z buttons in the spline menu and then try each one with each of the buttons in the align section. Also,from the looks of it you won't get a great result even if the spline direction is correct, the spline should be the shape of half the hull. From your pic it looks like you have only done the top section of the hull shape.
I've worked out how to do it in Max 3, it's probably the same in Gmax.
With the completed spline (example in pic) click the sub object button and make sure that vertex is selected from the dropdown. Select the 2 vertex's that you want the spline to occur around as shown in pic 1.
Select the spline option and then click the X button. You should then convert it to an editable mesh and perform a weld operation (set at about 0.001 should work) on the object to get rid of excess vertex's. You should hopefully end up with something like in pic 2 (converted to editable and vertex's welded of course).
