Choosing a knife is important, and you should consider several facts. First: do you take good care of your knife? Do you rinse it regularly with fresh water, keep it lubed, and sharpen it regularly? If not, don't buy just any old knife. Consider one of Spyderco's Salt Water Series knives (from $91.95), or, if you're willing to spend a few extra dollars, consider a Boye Cobalt knife, one of the finest series of knives we've ever come across.
Whether you buy one of Boye's folding knives ($129), or the Basic 3 ($360), you're getting a highly saltwater-resistant tool with a blade that stays sharp, and more importantly, doesn't dull easily even when cutting today's high-tech rope, which is notoriously tough on blade sharpness. Call it overkill, but when I'm racing to Bermuda this year, I'll be carrying a Boye Basic 3 permanently attached to my Spinlock Deckware PFD, and a Boye folder in the pocket of my foul weather gear.