MeatalSeadraMon, ruler of oceans was the first Dark Master
for digidestined to face ( Not counting that episode where all Masters
appeard at once, when digidestined ware saved by PixieMon ).
He was very strong and smart and using his minors almost defeated digidestined, which
ware saved by WareMon. However they couldn't even kill him, so WareMon
had to fight him himself. He secced to kill MetalSeadraMon, but
died himself as well.
About outlook: He looks just like MegaSeadraMon or any other SeadraMon's level,
but has almost all body covered with metal. SeadraMon looks like
a long snake and his outlook is actually based on an outlook of see monster
who usually appears in old legends.