Cruise line operator and Motley Fool Stock Advisor pick Royal Caribbean (NYSE: RCL) reported first-quarter earnings this morning, and the numbers suggest that cruise vacations are holding up well in a slow economy. Total sales improved 16.8%, while profitability surged ahead nearly ninefold on increased ship capacity and higher revenue per passenger.

The only negative for the quarter was those pesky fuel costs, which rose 53% to $158 million. Fortunately, booking trends were strong, and net yields -- an industry measure for net revenue per available passenger cruise days (APCD) -- grew an impressive 7.1%. Management also did a nice job of controlling costs, as net cruise costs per APCD increased just 2.9% despite the jump in fuel. Excluding the fuel prices, net cruise costs decreased 1%.

As a result, quarterly earnings of $0.35 handily beat analyst projections, but management lowered its full-year guidance on the expectation that current high fuel costs will stay that way for at least the rest of 2008. Earnings could hit close to $3 per share, but could also end up about flat from 2007 (when they came in at $2.82) if high fuel costs persist, or if strong booking trends catch a cold from a chilly economic climate

Royal isn't the only cruiser facing these trends. Archrival Carnival (NYSE: CCL) (NYSE: CUK) is in the same boat in regard to fuel spending, and Motley Fool Rule Breaker recommendation Steiner Leisure (Nasdaq: STNR), which runs the spas in many cruise ships, has traded down in sympathy, even though operating results still look pretty good. The key culprit is fuel costs, and while you have to credit management with doing an excellent job of running its business with the long term in mind, these stocks aren't likely to move until energy prices fall back below sea level. 

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Fool contributor Ryan Fuhrmann is long shares of Royal Caribbean and Carnival, but has no financial interest in any other company mentioned. Feel free to email him with feedback or to discuss any companies mentioned further. The Fool has an ironclad disclosure policy.