Your problem has 2 origins: the simpler is that the subsurf modifier should be after (down) the mirror modifier. The harder is that the mirror modifier uses as mirror pivot point the origin of the object, and your mesh is not in line with the axis. You can solve the issue in this way:
switch to top ortographic view (toggle Numpad7 and 5); rotate your mesh until is quite aligned with the axis; in edit mode select the center line and scale it on the x axis by 0 (S,X, 0, Enter); put the cursor aligned with this edge loop (Shift S, cursor to selected, then - in object mode - set origin to 3D cursor); select the edge loop next to the central one and dissolve it (X, dissolve edges).
It's always a good practice to mantain the origin of the object in the center of the scene during the modelling phase, then move objects to the desired position.