The Tohjo Falls are a magnificent site which attract thousands of tourists from Kanto and Johto a year; the sight of water cascading down from above is simply amazing. For a human or most pokemon to go swim over them is suicide (sadly, often quite literally; a few people over the years have given up on life and found it a sufficiently glorious way to die) and for a boat to fall there is a certain shipwreck. And yet as the water falls, if one looks closely they can see numerous Seaking riding it up and down for fun.
The ability of Seaking to navigate waterfalls was once unique to them and Goldeen (who seldom live long enough without evolving to learn how), although there is now a machine which can teach it to a few other water pokemon. When riding falls they do so slowly and safely, but when the move is used in battle, however, they swim at an extraordinarily high speed along a miniature waterfall of their own to ram the opponent. The difference in speed is likely due to the power needed to swim against the falls, which requires a great deal of power which appears to defy gravity.
This technique is not solely used for Seaking to show off. Not only does it allow them to navigate the vast rivers of the pokemon world and spread far and wide, but they are capable of carrying trainers and even (if in sufficient numbers) pulling boats slowly across waterfalls. For their trainers, the Tohjo Falls are not the great barrier which separates Kanto from Johto, but just another watery route.