Well, it would depend on whether it existed in history and just sunk into the ocean recently, or whether it was always submerged. In either case, the effect on the European economy and other things from the 17th Century onwards was pretty great and would affect all sorts of things in various ways. There might be many more Northern Europeans in Canada in Mexico, the conflicts between Britain, Spain, France etc during colonization would be different. The World Wars would be different. All sorts of things would be different. Too much speculation to say. Hopefully the trend towards idiocracy and massive resource exploitation would be reduced, but who knows?