IMO:How common is it for someone to commit suicide by drowning themselves?. I cant imagine two people deciding to choose that way to go. I know when i was going through dark years that drowning was never even contemplated.
I can see jumping off a bridge or cliff especially if one can't swim but walking into water or jumping off of a bank into the water I see as dicey.
Going under the water and when you first can't breath I think the bodies initial response would be panic and striving for air so you'd try to surface.
There's the rock theory but unless these rocks were somehow secured in their clothing, and that clothing was secured on them they could be taken out of pockets etc to allow you to surface.
I still can't see 2 people, no matter how close doing this at the same time successfully.
The bigger question if their bodies are recovered, which most likely we'll never get an answer to, is what was so horrific about their lives that they agreed that committing suicide was their only way out?
Nothing about the 2 women's actions makes sense to me, not that it is expected.