Werewolves get the power from the Moon as we all know. They are tan and transform into large wolves. All are born with a soul mate, one that is meant for them and only them. It was never declared that a wolf only gets another wolf as mate though that is usually the case, but since there are other supernatural beings in this world there is a chance for a rare pairing.
Mermaids are creatures of the sea, they are of the God Poseidon but grow strong by the sun. No more of that Ariel crap, mermaids can shift to human just as werewolves do. The water thing isn't true either, mermaids only shift when they want to or feel it necessary to. Mermaids get mates as well, though typically they only get other mermen as mates there are occasions that differ.
So what exactly happens when the Alpha of all Alphas and daughter of Poseidon meet unexpectedly?