I think being in the depths of water is an emotional thing. I had a dream where I jumped ship and sank all the way down to the sea bed....it was unsettling to say the least but that did not make me lucid. What could have made you lucid was the emotion of being in water rather than realising you were swimming underwater. Like I had a dream where it was emotionally disturbing..a panic dream and it triggered me to say to myself...."I AM DREAMING". It may be a key for you to relate that emotional feeling to dreaming...lucky you....

IMJ