Felt like I was falling off to the right...
I used to have the same problem: a $8 horseshoe spirit level may do the trick.
Else, just set up your tripod properly or use the camera's inbuilt virtual horizon sensor if you have one.
if you used a flash, you will get one patch of very bright area.... which imho, for landscape photo, you would like to avoid. Unless it is the kinda of light from the street lamp.
Your first photo probably will be good if there is something more "happening" in the sky. Overall, it is a nice photo except for the obvious tilt.
what you can do is to shoot at twilight when there are still light in the sky and the lights from the bridge, buildings and street lamp are turned on.