You can find some hidden bodies of water on google earth.
In your case find it there and go back on the history and see if it has been there a while.
I found a decent sized pond that feeds our lake that way. It is on private land and I am trying to find out who owns it to get permission
I also would try a worm and bobber. Frog is good idea too if you can cast to it. Senko or worm with light weight can be counted down to approximate the depth
I reccomend going with a partner and being very carefull if you wade in. Mud can suck you up.