Pulled each other under.....
When I was enlisted I remember DIs telling us during swim qual that when drowning, people will literally drown each other in survival panic and thats why it was so important to be competent in the water, especially for water rescues when trying to rescue a geared up and possibly injured Marine.
He said if they panicked and survival mode kicked in they instinctively try and climb you to keep themselves from drowning, effectively pushing you under.
Must be what happened here. Would have been fine id they separated and swam upwards, instead they panicked and drowned each other....
Sad.