That makes Sam freeze, holding Castiel closer under the water. While he'd had his suspicions, the way that Castiel had reacted to the way they'd responded to his text, to hear Castiel actually say it...

"You mean that he ignored your safe word." It's not a question. He knows what 'didn't work' means in this case. "Cas, I'm so sorry. I didn't...we didn't know. If we had, we'd have been there so much sooner."