Of course there is often opportunity coming out of catastrophe, though I doubt the displaced people would see it quite like that. In reality, sea level rise (barring any sudden shifts caused by ice shelves collapsing into the sea) is a slow process. Places will become uninhabitable in stages, first of all not because they are under water all the time but because the frequency of flooding is not economically sustainable. For some coastal populations that does not require a lot of rise from where we are today. Then you may have only 10s of cm of permanent water building up over decades and centuries. It’s a protracted process of inundation and it would be many generations before The Lost City of Venezia might be a tourist attraction, with an interim period where it is just a mess.