There aren't many tunnels that are inspected by boat - literally - but here's one with which to start a list.
The red brick Wath Road Tunnel was part of the Swinton & Knottingley Joint's line south from Ferrybridge, linking to routes heading west, east and south from the area around Wath-upon-Dearne. Passing through it was a chord from Dearne Junction to Mexborough West Junction on the South Yorkshire, Doncaster & Goole Railway.
At 62 yards, it is neither hugely significant nor especially impressive. It did though serve its purpose well, doing so from 1879 until closure came in 1965.