We have recently experienced Spring tides when the difference between high and low water levels are at their greatest. Thus recent news bulletins reporting flooding on the east coast.
The photo was taken from the Gateway as you intimated.
One of the reasons I took it was that the area at the western side of the harbour alongside the Prince of Wales pier is to be developed to accommodate a new marina and also a new lock cut into Wellington Dock.
The difference in sea level at Spring tides is in excess of 6 metres (20ft in old money). I am not an expert in marine matters and I am sure that others can explain it better.
http://www.visitmyharbour.com/articles/3154/spring-and-neap-tides-explanations-and-example