Yes you are right! You're talking about the Thames Tunnel built between 1825 and the 1840s I believe. It was thought to be the first ever tunnel built beneath a river. Coincidentally my x4 and x3 great grandparents lived close by the tunnel at Rotherhithe, both of the great grandfathers were 'watermen'. There were so few crossings of the Thames in the early 1800s so watermen and lightermen took goods and passengers across the river, especially from the many ships from around the world moored at that part of the river by the Tower of London. They did an apprenticeship for 7 years to enable them to do that. The Thames being tidal made it difficult to navigate. I know the area and the history well and it's fascinating to walk the streets by the river. You would enjoy it I'm sure.