Sapperton Tunnel is unfortunately not filled with water, nor empty (if it was dry I would have walked up to the gate across it, if it was full of water, I would have come back with a Kyack). the Coates Portal is very pretty. Saperton Tunnel is 3817 yards long, and when it was opened in 1789 it was the longest tunnel in Britain, and remained so untill a longer tunnel was built on the Huddersfield Canal in 1811. The tunnel fell into disuse around 1910, and in 1977 the Cotswold Canal Trust restored the Coates Portal, The Cotswold canal trust hope to restore the tunnel, and have it open through its entire length some time soon.
I will have to go back at some point for a better look round.