While Fools should generally take the opinion of Wall Street with a grain of salt, it's not a bad idea to take a closer look at particularly stock-shaking upgrades and downgrades -- just in case their reasoning behind the call makes sense.

What: Share of  Carnival (NYSE:CCL) sank 5% today after Morgan Stanley downgraded the cruise line operator from equal-weight to underweight.

So what: Along with the downgrade, Morgan analysts reiterated their price target on the stock of $32 per share, representing about 7% worth of downside to yesterday's close. Carnival posted market-topping third-quarter earnings yesterday, but weak forward guidance implying a sluggish yield recovery and cost increases prompted Morgan to make the call on valuation concerns.

Now what: Morgan now expects Carnival to earn $1.60 per share in 2014, representing a 28% cut to its previous view. "Once again, investors need to look at least two years out to make the shares look cheap, assume a perfect recovery, and that there are no 'structural' issues causing the ongoing downgrades," wrote Morgan analysts in a note to clients. Of course, when you consider Carnival's still-massive debt load, the risk/reward trade-off looks even less appetizing.

Fool contributor Brian Pacampara has no position in any stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.