Essex-based Thames Gateway Tramlink Ltd say they are working on a proposal for building a tram line including a tunnel under the Thames river:

" We, at Thames Gateway Tramlink, are very proud to be working on the KenEx Tram proposal. This exciting new project will provide an accessible, sustainable, public transport link between North Kent and South Essex, generating significant economic benefits to the area whilst also tackling increasing levels of road congestion. Furthermore, the project supports the increasing pressure for sustainable “Net Zero” transport solutions to combat pollution levels."

From the illustrations on Kenex project page, it looks like it would be an immersed tunnel.

https://kenextransit.co.uk/kenex-project/

An article at Londonist gives the whole picture.

https://londonist.com/london/transport/tram-under-thames-kenex-kent-essex-thames-gateway-tramlink