Frequently Asked Questions - Lost Disc
QA-LOS-1: My throw was headed toward an OB lake when it went out of sight, and we never found it. Do I play it as lost, or as OB?
If your group agrees that there is compelling evidence that the disc went into the OB lake, then you assume that that is what happened, and play it as OB. If there is uncertainty about whether it went in the lake, then you play it as lost.