Bad smelling watery discharge is mostly suggestive of vaginal infection
and caused by either trichomoniasis
or bacterial vaginosis
For confirmation, you may need to consult with gynecologist for pelvic exam & vaginal swab test.
Maintain genital hygiene properly, require proper anti microbial therapy along with vaginal suppository for complete cure and use condom if you are sexually active. In such cases, your partner should be treated.
Be well and take care.