Hi again,
Searching for "Nicholas II" + "Mar del Plata", in books.google.com, I found:
The United States and the southern cone: Argentina, Chile, and Uruguay
Arthur Preston Whitaker - 1976 - 464 pages - Snippet view
Two years later, Pellegrini presided, and the future Tsar Nicholas II of Russia was a guest, at the banquet inaugurating the luxurious Bristol Hotel, around which the high life at Mar del Plata revolved for the next half century. ...
books.google.com - More editions
It's a book published in USA, in 1976, and also speaks about Nicholas being at Mar del Plata in 1888. Unfortunately, the book is in snippet view, for copyrigth reasons.