Since I am a fan of cyberpunk, which often deals in transhumanistic questions, I often asked myself the same question. And I came to the conclusion that a permanent tail would be awesome, but I couldn't live bound in a wheelchair (by choice at least, hope I'm not coming of as ableist or anything). BUT if the technology would allow it I would probably get some webbed hands and feet (maybe even modify the feet further to give better water propulsion), and if I was able to get more extensive stuff I would love to have cybernetic gills or any method of extending my time underwater, like adding an internal air tank. And for the finishing touches I would love to get scaled skin at the legs or something like that, purely for the looks of it