A pure water/rain water.
Characteristics of Hamsodaka
दिवा सूर्यांशुसन्तप्तं निशि चन्द्रांशुशीतलम्| कालेन पक्वं निर्दोषमगस्त्येनाविषीकृतम्||४६||
हंसोदकमिति ख्यातं शारदं विमलं शुचि| स्नानपानावगाहेषु हितमम्बु यथाऽमृतम् ||४७|
Water naturally heated with sunrays during daytime and cooled with moon rays during night, well purified by the course of time and detoxified by the effects of Agastya (the star Canopus) is called Hamsodaka. This is available during sharada and is clean and clear. This water is beneficial as nectar when used for the purpose of bathing and drinking. (Cha.Su.6/46-47)