洗 puts 先 (first, also the sound) beside water 氵 — originally washing the feet specifically (ancient texts had separate verbs for washing hands, face, hair and feet; 洗 won the merger). Water first, everything else after.
洗 cleans the day: 洗手 (wash hands), 洗澡 (bathe), 洗脸 (wash face), 洗衣服 (do laundry — and 洗衣机, the machine that liberated it). Digital life borrowed it: 洗照片 (develop photos), and darker, 洗钱 (launder money — the metaphor is universal). 洗手间 ('hand-washing room') is the polite word for restroom you'll actually use.
Water 氵 first 先 — WASH.
洗手间 'hand-wash room' is the standard polite restroom word — more refined than 厕所, same destination.