My 22-month-old daughter absolutely hates getting her hair wet.
At bathtime, she complains (crying, but without tears) when I pour water on her head in preparation for washing her hair (even if I'm careful not to get the water in her face and ears). She doesn't mind when I wash the hair. Rinsing is the same. Wetting her hair by having her lean back into my hand and dipping the back of her head is the same...she's very emotionally uncomfortable in this position. (Once, I even put her on her back in the empty bathtub before turning the water on, and she panicked and got up very quickly.)
I imagine this goes back to something we must have done earlier - my wife suggested that we might have gotten water in her ears when she had ear infections. Either way, the real question here is, what can we do to help her overcome this?