Castiel flails his arms at Dean, sputtering in the water. It's more physical reaction than any real fear. It's a dream and he's an angel.

Knowing that, he turns to grab Dean, gripping him tight and... attempting to drown them both for a moment.

Under the water, he finds that if he stops thinking about breathing, he stops feeling it. Shaking Dean gently, he opens his eyes and tries to get Dean to do the same.