You think most of the worlds oceans aren't below 0 c? Just because they're below, doesn't mean they freeze; there is too much to freeze which means there are just random iceburgs.
When it's low enough (especially when the sunlight is in much less quantity in other portions of the world), it'll form pretty massive formations of ice (Antarctica, for example).
Additionally, I don't think most of the worlds oceans are below 0° C. It would be foolish to assume so seeing as massive bodies of waters retain their temperatures extremely well (they're very resistant to short but radical temperature spells); seeing how the average temperature on the surface of Earth is nowhere near 0° C, it would be a very illogical conclusion.
Pure H
2O freezes at 0° C, period. No exceptions. Even if it is part of a huge body of water, it's going to freeze. Oceans are resistant to freezing over for two reasons: One, they're salt water. Salt water's freezing point is lower than pure water (as iago already mentioned). Secondly, they're huge bodies of water. This has nothing to with the temperature that the water must delve to in order to freeze. It's for the same reason that an ice cube melts when you put in a cup of water. Heat (the diffusion of thermal energy from an object of higher thermal energy to an area of lower thermal energy). The entropy of a frame of reference specified in an ocean (1 Km
3) will reach a state of perfect entropy (randomness) given some amount of time (which depends on the temperature of the water to begin with) if you were to apply some force to a smaller portion of the water. This means that the temperature of the entire frame of reference must change, not just a tiny place where you applied the force.
However, if your frame of reference is really large (say an entire ocean), there will be random blotches of ice (ice burgs, like you said). There are obvious and alternative reasons that ice burgs exist, though. For one, a lot of them are still around from when the global temperature in fact was below 0° C. They're constantly melting small amounts into the water that surrounds them (following the law of entropy).
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