Abstract: An inboard marine engine flushing device for flushing the engine of a boat in dry-dock including a flexible, resiliently deformable preferably elastomeric conical shaped hollow member having a smaller inlet and a larger outlet. The outlet includes a water sealing surface which is sized to sealably engage around an engine sea water inlet fitting fixed to a boat hull. A preferably T-shaped hollow water supply conduit is also included having a first leg connected to the inlet end, a second leg connected to a pressurized water supply, and a third leg which is plugged or otherwise sealably engaged into a telescopically extendable and lockable elongated ground engaging member. The ground engaging member lockably retains the water sealing surface forcibly urged against the boat hull during engine flushing. The conical member is also bendably resilient to accommodate hull surfaces which are at a substantial angle to horizontal in the vicinity of the sea water inlet fitting.