Seadramon just got a solid win so now it’s time for him to accept the loss. Marcus is much stronger than any of Seadramon’s evolved forms and the gap between their abilities is simply too large to be bridged. Marcus and Agumon have handled much stronger threats in the past and they typically come out on top. Marcus wins.
Both of these Digimon aren’t exactly top tier fighters, but they can still hold their own. Whamon has his Neptunemon form and he is pretty good at close combat. Seadramon’s evolutions are all basically powered up versions of himself. His long range energy blasts become more formidable with each form and he can shoot numerous blasts at once. Whamon will have a difficult time trying to evade for long and he doesn’t have enough durability to claim victory. Seadramon wins.