RedactedNarrative: Unit 1 was travelling South parallel to I-95 on Lake Marion.  Unit 1 started having engine trouble so the stopped.  While trying to determine the issue, waves began coming over the stern of the vessel.  Unit 1 then capsized and began sinking.  Both occupants were in the water for 10-15 minutes before being recovered.  Vessel has not been recovered.  No injuries reported.