From ScienceDaily: Why Mercury Is More Dangerous in Oceans
Short answer: Methylmercury binds more tightly to the chlorides in salt, while the organic compounds it binds to in freshwater make it more quickly and easily broken down by the environment; so even though the overall concentration is less in oceans than freshwater, the little that's there ends up being more permanent, and thus carried on up the food chain where it becomes concentrated in the bodies of larger predators.
Short answer: Methylmercury binds more tightly to the chlorides in salt, while the organic compounds it binds to in freshwater make it more quickly and easily broken down by the environment; so even though the overall concentration is less in oceans than freshwater, the little that's there ends up being more permanent, and thus carried on up the food chain where it becomes concentrated in the bodies of larger predators.