This card's artwork shows "Torpedo Fish" under its belly and six "Cannonball Spear Shellfish" on its head. "Orca-Mega Fortress of Darkness" may be launching those as part of an attack (its effect obviously refers to the ability of shooting at monsters and spell/traps, using these two creatures) . Also, the monster swimming alongside "Orca-Mega Fortess of Darkness" is "Amphibious Bugroth MK-3".
These two monsters seem to be partners since "Orca Mega-Fortress of Darkness" is seen rescuing "Amphibious Bugroth MK-3" on the artwork of "Salvage".
The "of Darkness" in this monster's name refers not to its attribute, but to the darkness in the deep sea.
The fact that this monster is a Sea Serpent is unusual; Whales have no snake like characteristics, and are, in fact, Mammals. However, Sea Serpent is another term used for sea monsters which whales were often thought to be (i.e. leviathan is Hebrew for whale).