The Pacific is somewhat higher than the Atlantic at the Panama Canal. I can't remember the exact difference off the top of my head but I would guess somthing like half a meter. There was some concern expressed during debates about digging a sealevel Panama Canal because there would be a net flow from Pacific to Atlantic likely bringing Pacific animals with it. Nobody can know what the long term effects would be, but one can speculate that a flow of species from Pacific to Atlantic might have effects. The higher sealevel arises from the fact that the Atlantic is so much saltier though it is warmer. It seems the salinity dominates over the warmth of the Atlantic. Since the extra salt makes the Atlantic heavier, the tremendous force of gravity on this huge mass of water makes it a bit lower than the Pacific.