Phonosemantic compound. 氵 represents the meaning and 夬 represents the sound. Based on the original meaning "burst open". The meaning later shifted to "clear", "cut off", "decide" and "judge".
決 and 夬 don't sound similar in modern Mandarin due to historical phonetic changes. They were more similar in older Chinese.決 /*[k]ʷˤet/, /*[k]ʷˤet/ 夬 /*[k]ʷˤret-s/