And there could be some initial placement issues, as the rules require you to place them where you have an influence marker or pawn. And keep this in mind with Sea Serpent. From the rules:
"Sea Serpent must be placed on a port bordering the sea or river."
So IF you don't currently have an influence marker or a pawn on a port bordering the sea or river, you couldn't place a Sea Serpent. You'd have to instead try to immediately deploy the Behemoth.