Venice was built on marshy landfill in a lagoon and has sunk 23cm in the last century. This is the main problem here. It will continue sinking and there's no way to make that stop as you can't build stable foundations under an entire stone city built in the actual sea on top of mushy mud and not anything stable. Venice is and always has been inevitably doomed due to poor engineering choices made centuries ago. Same goes for much of New Orleans.

Interesting. It looks like the sea level has also risen by about 17.5cm in the last century - https://www.epa.gov/climate-indicators/climate-change-indica...

The fear is that the gates will need to be closed too often and for too long, damaging the lagoon’s ecosystem: https://www.nature.com/articles/d41586-018-07372-3