洗发剂 and 洗发水 difference
I have some more:

洗发水

洗发剂

洗发液

洗发露

洗发乳

洗头膏

洗发精

洗发液

洗发香波

They all mean shampoo. If there is any difference, that might be frequency or regional preferences. The last one is a loanword (香波)