Could an iceberg with a few seal families on it have drifted from the Arctic to the Antarctic? Presumably in the tropics the seals would have had a bad time and gone hungry and overheated, but once they got to cooler waters again, they would have been surrounded by food they love with zero competitors.
Life travels on the ocean a lot farther than people realize, and often species you'd find improbably.
Ants spread around the world by hitching rides on coconuts or waterbirds. This now means there's something like a global war among various ant colonies that invade each other this way.
I thought Wind, Wings and Waves to be a pretty interesting description of how species made it to Hawaii. Basically it either gets picked up in storms or large clumps of land make it across the ocean. Life only has to make the journey seldom or almost never and then take hold once to get to a new place.
It's my understanding the ant spreading was more related with increase in navigation than birds/floating. that'd be why the Argentine ant has spread everywhere in Europe in the last century rather than being ubiquitous since the dawn of time.