Offering water to sun is one of the most important rituals in Hinduism. This is good for health and for mental peace. Here is a mantra for offering water to sun.
The best mantra for offering water to sun is the Gayatri Mantra:
ॐ भूर्भुवः स्वः तत्सवितुर्वरेण्यं
भर्गो देवस्य धीमहि
धियो यो नः
प्रचोदयात् ॥
Om bhūr bhuvaḥ svaḥ
tat savitur vareṇyaṃ
bhargo devasya dhīmahi
dhiyo yo naḥ pracodayāt
Om bhūr bhuvaḥ svaḥ
tat savitur vareṇyaṃ
bhargo devasya dhīmahi
dhiyo yo naḥ pracodayāt
The mantra should be chanted once facing the rising sun and offer water. Meditate for 15 minutes facing the rising sun.
But if you want to try out any other mantra other than this the best option is
Om Arkhaya Namah:
ॐ अरखाया नमः
ॐ अरखाया नमः
This mantra should be chanted 108 times facing the sun and while offering water.