As far as science is concerned, there is no Swampert in Sinnoh's Great Marsh. The lack of any Mudkip sightings there alone is proof enough of that fact, but if pressed, frustrated scientists will also point to the temperature being too cold for Swampert to survive in the wild, and the water which attracts poison types to the region is more acidic than any known Swampert species can endure
According to the locals, there is a single Swampert in the area, a male who has lived there since before human habitation of the land and is still alive today. He is thought to be male, which is why there are no Mudkip, but according to them many of the unique characteristics of Great Marsh pokemon developed not in isolation, but because, like a monstrous version of Zeus, he has sired countless children over countless generations with many local pokemon. Once, he was worshiped as a god, because he was ageless and thought to be a unique species, and fed on lavish sacrifices of human and pokemon alike. But when books from Hoenn found their way to this remote region, he was denounced as a fraud and wounded in a revolution.
Since then, the Swampert of the Great Marsh has refused to show his face in daylight. He digs countless deep traps in the marsh, which are claimed to be the reason why few can journey from one end to the other without getting stuck, and feeds on those hapless pokemon who do not climb out in time. A few people caught out there on late nights report catching a glimpse of his face, and some even claim to have been rescued by his mercy, but all know stories of children who went into the marsh at night and never returned.