What your character doesn’t know … can kill him.
A powerful way to pump suspense into your story is to reveal more to your reader than you do to the character.
In the above image, compliments of my superb and incredible friend, Elai Thiessen, the man in the boat (your story’s protagonist) sees only a mesmerizing woman — he doesn’t see the body of bones clasped in the end of her tail. But we do. We know what’s at stake.