Downwelling

The arrangement of the continents would certainly make circulation patterns different than they are today. For example, it seems like on the surface, there wouldn't be so many gyres, since there was only one giant ocean basin. There might have been more east-west movement and perhaps fewer north-south currents, since there were fewer continental barriers to flows driven by the prevailing winds.

If the air was warmer at the poles, then the water would be too. And without cold water and ice formation at high latitudes, there would be little downwelling and not much of a global conveyer belt in the deep sea. So all in all ocean circulation would probably have been much weaker than it is today. Oxygen in the deep sea would have been used up, and without strong downwelling, there was no way to replenish it.