Unless the population of Fukushima is suddenly going to develop water powers in the MCU, that throwaway line really doesn't hold up. Radiation doesn't rewrite genetics, it may have triggered inactive genes, but being the root cause is completely off the reservation.
The simplistic cartoon logic of the science at play here is distracting to me, but clearly my problems with the background story and resolution aren't shared by everyone, that is fine. I'm happy you enjoyed it all more than I was able to.
Hopefully the next one is more satisfying.