I haven't seen anyone open the civilopedia page on tourism yet, despite some going for culture victories.
Marbozir hovers over the other civs in the tourism victory tab every now and then, this shows some number for tourism (which I couldn't derive directly from his tourism value and the multipliers in place) and it also shows tourism acquired over the course of the game. So apparently it is something you build up. This is also supported by the fact that Spain's domestic tourists and tourists visiting his country did not change when he captured 4 out of 5 Spanish cities. In the end he nuked most Roman cities, but the Roman tourists apparently didn't mind that and still happily visited his beach resorts.