I don't understand how you can get the horizon so crooked when using a tripod.
I assume you have all the time necessary to level your camera. This will also help you to max out your full sensor size. If you always have to rotate your image you will lose some parts of your image, something you don't want.
You also need to inspect and evaluate the lighting conditions of your night scene. If you have only one light source in your scene (moon, street light etc.) it will get tough to create a nice night shot.
Ideally you need some light to illuminate your scene to some extent. One light will create one hot spot in the image which could also create some reflections (eg on the water surface) but the rest of the scene might end up too dark. There you might have to work with HDR but that's not your goal.
Night photography becomes a lot easier when you have more lights illuminating your scene. Spreading the light sources all over your frame will balance out the lighting.
Bitter Jeweller posted some great night shots before:
http://www.thephotoforum.com/forum/photography-beginners-forum-photo-gallery/203036-i-high.html
There you can see what a lot of lights do to a night scene.
A single or very few light sources are much tougher to master than an illuminated city.
Just understand what is possible with the scene that you're looking at and don't expect the impossible.