William Blair analyst Sharon Zackfia downgraded Carnival (CCL) to Market Perform from Outperform following this morning's Q2 results. The company lowered expectations for the remainder of the fiscal year, with constant-currency net yields now expected to be flattish to slightly down in Q3 and down in Q4, translating to full-year net yields of flat versus prior guidance of up approximately 1%, Zackfia tells investors in a research note. She points out that Carnival's flattish yield expectation compares to underlying guidance for mid-single-digit gains at Royal Caribbean (RCL) and Norwegian Cruise Line (NCLH). The analyst attributes Carnival's underperformance to its nationalistic brands and greater exposure to Europe. The ongoing geopolitical uncertainty in Europe and today's lowered guidance is likely to put the stock in a "show-me state and limit shareholder returns," Zackfia contends.