I have herniated disks and degenerated disks along with other things and i am unable to use my legs at all, but I still swim around 20 lengths most weeks. mostly breast stroke with the odd one on my back flapping my arms in and out to the side. I take a woggle with me to hang on to when i need a rest because my legs float around like octopus legs!. I also do stretching and 'walking' exercises in the pool too with the help of a carer.
the rules i tend to use are :
take it slow,
if it hurts , stop
remember to breathe
stop before you get tired (I still need to get home and upstairs into bed!)
relax and enjoy just being in the pool.

I never take pain meds for three hours before going swimming, that way i am totally aware of when i have done enough. taking pain meds before exercise can fool you into doing too much and over exherting your self.