शैत्याच्छयनयोगाच्च शीति वारि प्रचक्षते ।तत्पानाद्रक्षणाच्चैव शिपयो रश्मयो मताः ॥
In plain words
Water is called śī on two counts: it is cool, and it is where things come to rest. And the rays are called śipi because they drink that water up and because they guard it.
Close to the Sanskrit
From coolness and from connection with lying-down, they call water 'śī'; and from the drinking of that and from protecting, indeed, the rays are held to be 'śipi'.