I am not sure whether this has been suggested before, but one of the problems here is that while the Med is classed as 'Tideless' (a tidal range of less than 1ft/0.3m), Venice itself has a tidal range of around 1m, so it seems likely that even if the whole of the Med were dammed (and it would mean blocking the Suez Canal also) then there might still be a tide in the region. Also if the sea levels rise by 1m then there is no possibility of draining water out of the Med at low tide in the Atlantic, so what then?

Having said that, these are just my first thoughts and it is not worth rejecting an idea such as this out of hand so perhaps a more detailed study would be of use, including a look at the political issues (such as who pays - Italy as Venice is the main beneficiary or whoever gets the opportunity to sell the generated electricity which might be Spain or Morocco, or even UK through Gibraltar?)

I would also say I do not believe the cruise ships share any of the blame for the flooding, though I am aware of many other issues they are causing and do feel they should be banned from Venice, just not on a spurious excuse.

Alasdair