Yes, If plannig a water birth TENS can be a great comfort measure from the begining of early/active labor up until you decide to enter the birth pool (usualy around the time of transition, or 9-10cm dilation). TENS units are not waterproof and cannot be used in the shower or during the time you are in the birth pool
The electrode pads are reusable, and can be placed back on to dry skin if you decide to exit the tub again during labor.
The unit can also be used in postpartum to relieve afterbirth aches and discomfort.
Optionally you can purchase additional electrode pads in case they lose adhesiveness during birth or for further use postpartum.