Deutsche Bank Alex Brown, Salomon Smith Barney and Credit Suisse First Boston have all cut their ratings for top cruise group Carnival Corp. Deutsche Bank Alex Brown and Credit Suisse First Boston downgraded the company to buy from strong buy while Salomon Smith Barney cut ratings to outperform from buy.The downgrades follow Carnival's announcement that second-quarter earnings look weak, in part due to higher fuel costs. First-quarter earnings were in line with Wall Street estimates.