At this point, let's take a look at what the model should look like so far. This screenshot is of the Render Preview, with "Render Backfaces" off and the option below that, "Render wireframe", on.
Note that I have created polyfaces for exactly one quarter of the model.
Now, we need to tell Rhino what exactly we want to mirror. Since the only surfaces in the scene are the ones we want to mirror, we can use a shortcut to select them. Make sure you don't have anything selected, then click on the Select All Surfaces button towards the top. It will (surprise!) select all the surfaces in the scene, and leave all those polylines alone. Don't worry - we'll deal with those lines later.
Now we can mirror the surfaces. If you didn't have anything selected when you click on the Mirror tool, it would ask you to select the objects you want to mirror. 
Since we already have the surfaces selected, it just skips that step and asks for the location of the start of the mirror plane. To make things easy, deselect any checkboxes in the OnSnap toolbar, and make sure that "Snap" is selected. Move the pointer to the grid line dividing the two halves of the model, either in the top or front viewports, and click. Now move the pointer along that grid line, making sure that the mirrored surfaces line up the way we need them to, and click again.
Next, repeat the same process to complete the model's hull, starting from the Select All Surfaces button. Mirror the top half of the model to the bottom half, and then do another Render Preview to make sure that everything is ok.
Your model should look like the one in the picture. If not, ummm... go back and do what ever you did wrong right this time! ;)