Water is a gift of God that makes life possible and yet millions of people do not have access to safe drinking water, and rivers, seas and oceans continue to be polluted, Pope Francis said.

"Care for water sources and water basins is an urgent imperative," the pope said in a message Sept. 1, the World Day of Prayer for the Care of Creation, an observance begun by the Orthodox Church and now celebrated by many Christians.

With the world day 2018 focused on water, Pope Francis drew special attention to the more than 600 million people who do not have regular access to clean drinking water.

"Access to safe drinkable water is a basic and universal human right, since it is essential to human survival and, as such, is a condition for the exercise of other human rights," he said, quoting from his encyclical "Laudato Si'" on the environment.

"In considering the fundamental role of water in creation and in human development," he wrote, "I feel the need to give thanks to God for 'Sister Water,'" as St. Francis of Assisi said. Water is "simple and useful for life like nothing else on our planet."

Fulfilling the Gospel mandate to give the thirsty something to drink involves more than individual acts of charity, although those are important, he said. It also involves "concrete choices and a constant commitment to ensure to all the primary good of water."

Believers have an obligation to thank God for the gift of water and "to praise him for covering the earth with the oceans," Pope Francis said. But they also have an obligation to work together to keep the oceans clean instead of allowing them to be "littered by endless fields of floating plastic."

Thinking of oceans and seas, also led the pope to think of the thousands of migrants and refugees who "risk their lives at sea in search of a better future."

"Let us ask the Lord and all those engaged in the noble service of politics that the more sensitive questions of our day, such as those linked to movements of migration, climate change and the right of everyone to enjoy primary goods, may be faced with generous and farsighted responsibility and in a spirit of cooperation, especially among those countries most able to help," he wrote.

Pope Francis also offered prayers for people who fish and others who earn their livings at sea, for those who minister to them and for all the scientists and public policy experts who help the public recognize the treasures of the sea and work to protect them.

And, as the Catholic Church prepares for a world Synod of Bishops on young people, he urged Christians to educate and pray for the young "that they may grow in knowledge and respect for our common home and in the desire to care for the essential good of water, for the benefit of all."