My 12 year old AS son hates showers - is terrified of them
He was able to explain to us that a shower feels like hot needles and it sounds like screeching, so, he has a bath instead. He and we are a lot happier, takes longer but less stressful for us all.
His twin sister (also AS) loves the shower and regularly uses all the hot water -
My son still does the bath thing too. The shower scares him. As long as he gets cleaned, its good with me.