There are two of these tunnels, one dry and lit, the other totally dark and wet.
We chose the one with water flowing through it.
It is narrow: two people can not pass each other without considerable difficulty. It is just five feet high in a couple of places, so I had to stoop in those places. the depth of the water is from a few inches to two feel. It is gently flowing, so relatively fresh.
If there are people in front of you, be prepared for them to stop to take selfies from time to time!
I would say a torch with fresh batteries is essential. Preferably a head torch. Oh, and a waterproof camera if you are concerned about it dropping in the water! Otherwise be careful!
One MUST wear shoes, as the paved stonework under foot is loose in places. Trainers are okay, but of course they will be wet afterwards. We used jelly sandals purchased for use on the beach. They were perfect. We wore shorts which limited how wet our cloths got. There is a changing room, but we did not use it.
At the end of the gently twisting tunnel are a few gently slippery steps leading out to the small reservoir pool and daylight!
Wow! what an unusual experience! Make sure you know the history of the tunnel before you go as it makes the experience even more interesting.