Low-light city landscape photo problems

As others have pointed out your exposure settings were not ideal. It also appears to me that your focus was not spot on or there was too much haze/humidity in the air for the shot to be worthwhile. The light pole in foreground is also a compositional problem but maybe your intention was to deal with it in post.

Digital capture is best when the sensor receives the greatest amount of light possible without reaching saturation. ISO does not affect the amount of light that falls on the sensor, it amplifies the signal. Aperture and shutter are the only two variables that affect how much light the sensor receives and in this case your aperture at f5.6 is fine but then you should be at a much longer shutter speed than 15 seconds. Use the histogram and blinkies option to push the exposure as far to the right on the histogram as possible without significant blinkies when shooting raw and then process the file to taste in post. Doing so will give you the best, cleanest file to work with providing the highest degree of flexibility in editing with the lowest possible noise.

As for focus I would use manual for a shot like this and magnify to the max on the LCD or in the EVF to ensure proper focus. Shooting distant landscapes over water can also be problematic depending on the relative air/water temperature.

Compositionally you'll find what you like after a lot of trial and error.

I hope this helps.

Bob