Hi there!
I used a TENS machine with my first delivery. I had it on for about five hours. I feel it helped me through the early stages as it gave me something to focus on, giving the boost button a press on contractions made me feel like I had control over my labour. In the last stage of labour I asked for it to be removed as it was bothering me and by that point I preferred the gas and air, which is fantastic by the way, and was the only form of pain relief I had with my second child! I would definately recommend a TENS machine as it poses no threat to the baby and you can just take it off if you don't like it.
I have no experience of a water birth but my advice would be to listen to your midwife as they will promote your wishes as much as possible as long as you and your baby are ok.
Best of luck with everything.