The Sea Witch Pub stood along the Greenwich Riverside Path, facing the river, until it was bombed in the Second World War and was subsequently demolished. It stood just where the path turns abruptly left to go round Morden Wharf - until five or six years ago this was the site of the Amylum Laboratory building, which strangely echoed its size, shape and layout.
I wrote an article about the Sea Witch for Bygone Kent a few years ago and it can be found online at https://greenwichpeninsulahistory.wordpress.com/2013/07/30/two-vanished-greenwich-pubs/ and I understand more research has been done by others, since.
We have now been sent two pictures of the base of a tankard which has clearly come from this long vanished pub. They came from Alex of the Brightwater Brewery which is in Claygate, Surrey. It occurs to GIHS that there must be more like this out there - so - if your grandpa is using a Sea Witch tankard to keep his used paint brushes in, please let us know (we also think that there are collectors out there willing to - er - pay)