Probably an insane amount. You would have to reinforce the buildings internally so they could withstand the additional forces. Might be cheaper to build a new Venice.

If sea level rise does ever seriously get out of hand, that is my prediction for parts of Tokyo and New York. I strongly suspect that some coastal cities would rather float than move.

New York 2140 is an interesting book examining what might happen to those coastal "drowned" cities, especially those buildings caught in the new intertidal zones.

https://www.goodreads.com/book/show/29570143-new-york-2140

That looks interesting, I might grab a copy when I have made a dent in my current pile.

Some architects are already proposing to build floating apartments off the piers in Manhattan - https://www.dezeen.com/2018/02/09/pier-40-manhattan-new-york...

One kind of nice/kind of weird part of it, is that it's mostly interconnected slices-of-life without a huge overarching plot (though that's not entirely true, either) -- that has made it quite good as a pick-up-put-down book: handy, considering it's length!

This is already happening in Amsterdam