I had a sim like that a few weeks ago. Her thalassophobia got so bad that she refused to do even the most basic things involving water, such as laundry and washing dishes in the sink. Showering still worked for some reason. I don't know where it came from or where it went, but one day it was just gone.
She did go swimming in the ocean a couple of times because that was the only way for her as an elder to keep in shape and build the fitness skill without dying, so exposure therapy might indeed have something to do with it.