Mantra to be recited while taking bath in home or holy bath in rivers etc. 

Gangeycha Yamuneychaiva godhavari saraswathi
Narmadey sindhuh kaveri Jalesmin sannidhim kuru

The literal meaning of this mantra is: "In this water, I invoke the presence of divine waters from the rivers Ganga, Yamuna, Godavari, Saraswati, Narmada, Sindhu and Kaveri. These rivers are considered as goddesses and I pray to them for considering me for their blessings. 

God has given us a beautiful life to live and has created this beautiful world. There are massive rivers and oceans in it. Many rivers like, Ganga, Yamuna, Saraswati etc have their own significance. Taking bath in these rivers is considered auspicious. So, reciting this Sloka/Mantra with a sincere heart brings you the benefit of taking bath in these rivers.

Gayatri mantra is also a common to say in the morning when taking the first bath, especially at sunrise.