Seadramon is Betamon's Champion form. His rookie form is Betamon.
Digimon Adventure 02
He first digivolves when the DigiDestined, who are at Digitamamon's restaurant, are attacked by a Dark Spiral-controlled Gorillamon. Seadramon then attacks Gorillamon and drags him underwater, until Togemon destroys the Spiral. The Samurai of Sincerity Michael and his Seadramon show up again later when Digimon begin to appear in the Real World around Christmas time and battle a Cherrymon at Rockefeller Center with help from ExVeemon, Lillymon and the other American DigiDestined's Digimon.