Try not to stop down on DSLR more than F16. F16 is as small as you want to go on aperture. We usually do F8-F11. If we want longer exposure times, we will use ND filters to lower the amount of light entering the camera. If your aperture is too small on a Digital SLR, you get some sort of distortion called
small aperture diffraction.
You can read up on small aperture diffraction
here.
What you want to do in night long exposure photography is:
1. lower your ISO as much as possible (ISO100 for me)
2. aperture around F8-F11
3. Tripod mount.
4. White balance on Manual
5. Manual focus on lens.
6. Use remote shutter trigger (wired or wireless)
7. Use M mode and Bulb mode a lot.
8. ND and GND filters.
9. If you want to remove reflections, CPL use is ok but remember it lowers the exposure by 1.5 to 2 stops. So adjust your shutter speed accordingly. Usually, for night long exposure, we actually like the reflections. It gives the water character.