How did you know you almost blacked out? I'm most terrified of the stories that you can fade without knowing

I made it back to the surface and started feeling that familiar dizziness / light headed sensation. Not sure how to explain. Similar to when you stand up too quickly sometimes and feel like you're going to pass out.

I started swimming towards shore, pulled myself on the rocks there and rested. It felt weirdly wonderful. Warm rocks heating up my body, and a great sensation of calm. Only after a few minutes did I understand the gravity of what had just happened.