Left water radical - the indexing component, and a perfect mnemonic: when you're thirsty, you crave water. 氵 puts 渴 alongside other water-state and water-need words: 河 (river), 海 (sea), 汗 (sweat), 流 (flow).
Right 曷 supplies the sound (hé → kě, h/k velar drift). Phonetic family: 喝 (drink, mouth + 曷), 揭 (uncover), 谒 (call on), 葛 (vine). The 渴 / 喝 pair is especially neat - same right side, different radicals: water-need versus mouth-drinking.
