The Seebühne, a floating opera stage of bewildering proportions rises every summer from Austria’s Lake Constance, the centerpiece of the annual Bregenzer Festspiele (Bregenz Festival). It has staged productions such as Verdi’s “Aida,” Giordano’s “André Chénier,” Bernstein’s “West Side Story,” and next year will be Mozart’s “The Magic Flute.”