QA-OB-5: My throw landed next to an OB creek. It’s hard to tell whether the disc is in the creek or not since the edge of the creek comes up into some mud and grass. Another player went up to my disc and pushed it down to see if there’s water underneath.

No. Note that the interference and position rules are written in terms of a disc being moved rather than merely touched. The other player did not change the location of your disc. In fact, a disc must sometimes be manipulated in order to determine its status or whose it is. If you move your possibly OB disc, it is automatically OB. But there is no corresponding rule that makes it in-bounds if someone else moves it. If that happens, you restore your disc to its approximate position as agreed upon by your group.