To be honest I think they don't know COD or MOD right now but that's just me. I think that they are going forward as a suspicious death and handling it as a crime investigation because that might be their protocol. It is for a lot of LE's.
I won't get into the details here because it's so soon after her being found but having had lived in Central TX we always had many many people that drowned in the area lakes. Both in summer and winter.
GRAPHIC WARNING:
So without going into too much detail I remember that it does take some time for submerged bodies to come up on their own. It takes longer if the water is cold (not cool) and if the ambient air above the water is cold too.
The average I saw was that a submerged body would eventually rise to the surface within a week, give or take a day or so. If it was in the summer. That's why I'm not assuming she was weighted down. She may very well have not been.
Another thing I observed, even if the body is in the water for a short time (such as a week) there will be obvious and feature altering bloating. There was no other word to use for it, skin slippage as well. So that would make it incrediably difficult for LE to look at remains and state that there were any obvious injuries because of that and because the body can be subjected to injury postmortem while under the water if there are things down there as well as any animal life underwater.
IMHO it's impossible for me to even speculate, personally, because of so many factors. KWIM? Just sharing what I learned from the countless drownings on those lakes when I lived in CenTx. All JMHO