This is exactly what you're looking for, I think!
These were taken by a rail engineer and posted in the Mersey Railways group.
It's not a 'lake' exactly but a header tunnel/sump. Apparently a rowing boat is indeed used for engineering inspections..
There's a fantastic report written by the engineer who built the tunnels, published in 1886. Lots of detailed diagrams and descriptions: The Mersey Railway
These were taken by a rail engineer and posted in the Mersey Railways group.
It's not a 'lake' exactly but a header tunnel/sump. Apparently a rowing boat is indeed used for engineering inspections..
There's a fantastic report written by the engineer who built the tunnels, published in 1886. Lots of detailed diagrams and descriptions: The Mersey Railway