Inshore lifeboat launched at 2003 to report of small RHIB aground in the Swale with 2 persons on board. With the tide still dropping, the ILB proceeded at best speed to the area, limited contact was made with the casualty craft on VHF, but the ILB could still not see the casualty, eventually, the craft was located on the mudflats off Elmley. One crewmember attempted to walk ashore, but due to the soft and deep mud, returned to the ILB, after a lengthy discussion with the UKCG and the occupants, the occupants reported they had warm clothing and provisions and were happy to wait for the flood tide and would keep in contact with the CG. ILB released at 2140, returning to station at 2215. NW 4.