If the map is accurate, it looks like they entered the water in a controlled fashion, including the initial loop. If the horse did step off an underwater ledge, they could immediately have been in trouble both of them, especially if she was still tethered. Free of each other, they might have been able to save thineself, recover and reach the shore safely. Now, if that chaos was causes by wild life that forced them into the water and followed them, that too could account for what we see. There's a reason the horse (with the phone) didn't swim directly back to shore and IMO that's key to what happened here. Horse was either fighting against a foe or fighting against the rider, both of them flailing.
Once the horse broke free, it was able to make it back to shore and found a way back to civilization...
Is there a way for laypeople to know how much time elapsed in the water? If the horse was thrashing, it might have led to a drowning, held under too long, or all that time, Meghan could face been struggling to separate herself.
The waiting has to be pure agony.
Jmo