There was truth in the assumption that Castiel's vessel was being flooded by the Leviathans at that moment. He struggled against their hold, screaming and roaring at them in a horrifying manner as they injured his family, his Winchesters. He gained slight control for one second after hearing Dean's voice, questioning the Leviathans and taunting them about his vessel. They squashed Castiel back into hiding, but they weren't able to stop the look that he sent at Dean.
This is not goodbye.
His eyes screamed the words to Sam, Bobby, and Dean as the Leviathans won once more. 'I'm sorry... Dean... Sam... I've failed you,' Castiel thought, growing weaker by the moment. He could feel his strength failing and he struggled but it was a losing fight from the start. There were just too many of them.
The Leviathans were in a frenzy themselves, excited to cause devastation on this planet after being caged up in Purgatory for endless years. Castiel grew sick as he heard their thoughts and tucked himself away, protecting his mind and soul from them. They lead the body of Jimmy Novak to the Municipal Waters.
Castiel stumbled deliberately to stumble them, the last thing he could do. He could hear the others approaching as fast as they could, but they were still too slow. He walked into the water, feeling his family's gaze on his back as he had one final thought.
'Wait for me, Dean.'