Exact same thing happened to me just over a year ago - on a marked park trail in a provincial park. We came across a marsh and thought nothing of it, we figured since it was a marked trail it would be safe. We were wading through water about waist deep and suddenly my horse went down right from underneath me. I managed to get flung free, and my friend's pony just got her front legs in when I started screaming for her to get back. In the struggle, Zierra managed to turn herself around and be facing the way we came but quickly exhausted herself and just laid down. We stayed with her for half an hour, but dusk was coming quick and nobody knew where we were. I had to grab the little pony and gallop 7km back to the ranch to get help, and then gallop back with Shay-la on her green 3 year old Clydesdale/TB! Those two horses sure delivered for us that day!
It took us an hour and a half from the time I left, and during the time, my brave little Arab just regained her strength while Ashley held her head on her lap, talking and singing to her. She said she finally moved her head, and Ashley got out of the way, and she violently just exploded out of the bog. And then waited for Ashley to get on her and carried them both out of the marsh!
Absolutely most horrifying day of my entire life. I really thought I was going to lose her, and NOBODY would come help us.
Where she's laying is waist deep water, but there was so much marsh grass, you could actually sit on it and be kept out of the water.