This is an experiment to see what happens if the sea was one day 5m higher. The docks are all reclaimed land and are quite vulnerable, but the old city (Below Bar) is much higher up.
5m sounds like a lot, but normal tides in Southampton seem to be up to 4m variation (which I assume is +/-2). Predictions on sea level rise by 2100 range from 50cm to 200cm. One bad storm surge could render the lowlying parts of our city unusable. My back of the envelope maths is:
1m sea level rise + 2m high tide + 2m storm surge (last happened in the UK in 2013). https://noc.ac.uk/news/storm-surge-hits-uk-coastline
OK, I know that higher end of those predictions is only 1-5% chance that's not odds normal people would take of a massive disaster.