Tourists suddenly stop coming.

I've got the same problem. It seems the game is just not well balanced for resort cities. I've replayed several games just to see if the situation reproduces, and yeah it happens every time. So it seems you just nearly cannot play resort alone, without other players' cities bringing you tourists.

In the last game, I have three gambling house, three sleek casinos. I've built Expo Center which is full every day, and that is still not enough to be in green :-). So I've had to build a financial department to borrow 3x100k credits to get a Pro Stadium. And the stadium events are nearly in green themselves, but they bring people into the city. And still, the city with 100k population is like -12k$ in red for half a day, and +12$k in green for another half.

Ooh that looks soooo stupid Maxis :-).