Ice increases Earth's albedo, which will reduce global temperatures, causing some of the water to re-freeze as local temperatures dip back down to pre-industrial levels. Colder seas are also smaller (the NOAA cites ocean temperature increases as a key aspect of rising sea levels)
As for the cost, that was never in the scope of the discussion I had with Garnier Fructis, so I left it aside. That said, flake ice machines already can use seawater to generate ice (they're used by the fishing industry to keep products cold on-site) and are built to produce industrial quantities ("snow" machines for ski resorts, for example, are flake ice machines.)