[uber nerd mode]
First we need to define Zombies: Traditional Zombies (referred to as REAL zombies for this discussion) are reanimated corpses. They have no requirement for oxygen or water, or any food other then brains and flesh, which the reanimating agent (most often a virus) causes the corpses to crave. Real zombies would only be “fast” for the first day or so following reanimation, after which time, muscle decomposition and rigor mortise would cause the characteristic “Zombie Shuffle”. Real zombies would have no trouble reaching an island. According to leading zombie researchers, many small outbreaks of Zombies are caused when a zombie that has been wondering around on the ocean floor makes its way to a beach and bites someone. The only water defense that would be practical against real zombies would be a fast moving, waist deep stream with significant enough volume to knock down a less than average coordinated human. The current would then have to be swift enough to carry the corpse a significant distance downstream.
Another type of creature has appeared in recent fiction, which has been mistaken for zombies, but they aren’t. In the film “I Am Legend”, infected humans were presented (they were actually more like the traditional definition of vampires). These were mistaken for zombies, but were alive, and in this case had accelerated heart rate and breathing. Zombies have neither. In “28 days Later” and it’s sequels, infected humans were again presented in a Zombie movie type scenario, but they were not zombies, they were just infected humans that were really REALLY pissed off.
For these infected human situations, they would have as much trouble crossing water as a normal human, but without the reasoning and problem solving abilities required to use a boat.
Yeah… I know… [/uber nerd mode]