Instruction for 2.80/2.81:
Go to Edit mode, go to ortographic projection [5], set your camera to be above the ship [or below] [Scroll + Left Alt], select Knife [K]. set one cut point outside your model, press C to align your Knife to axises, press K for knife projection, drag your line through the middle of Orion's ramp, click on the other side of Orion [in front of the ramp] to set your cutting line, then apply it by pressing Enter.
Now go to Z [wireframe mode], toggle face selecting mode, press B for rectangular selection and select all the stuff along the knife projection edge. Prese Del, and select Faces.
Now your Orion should have nice, straight cut and hole. It's good.
Move all the geometry on one side of Blender Y axis [the green line]. You probably want to keep turrets on their positions, so go to edit mode, select everything and move them on one side of Y. If you made your knife projection from above, you want to move your Orion to the left. The line of your knife projection should be aligned with Y axis. Now make mirror and profit.
Before you add mirror, make sure that mesh is properly centered. If you moved the mesh with turrets in Object Mode, the pivot point [orange dot] was moved with it so mirror will not appear correctly. Make sure that your pivot point is on the Y axis as well. It's best when it's on [XYZ] 0.0.0. You can manipulate pivot point by using Transform Window and 3D cursor option. Move your 3D cursor to 0.0.0, then select your Orion in Object Mode, then hit Object -> Set Origin -> Origin to 3D Cursor.
Aww... And you may need to redo turret layout.
If you set all materials and smoothing setting correctly, your catamaran Orion should be ready for export.