The Beast Master heroic ability includes +2 Positioning against Beasts.
Does this include Monsters that happen to be Beast-like (or are you meant to make Monsters distinctly different from living things)?
When the Antagonist creates an opponent, it must be classified as Man, Monster, or Beast. If it's classified as Beast, the Beast Master trait applies. If not, it doesn't. There's no in-between or judgment call.
When you create a giant sea-serpent, you can decide if it's a Beast or a Monster. Each type has its own special advantages and restrictions (see page 92).
That's the way I was leaning, but I wanted to be sure.
By the way, just as I read the word Sea Serpent, a great peal of thunder rang out here. The Forge comes with sound (and weather) effects!