when at the river why did george tell lennie not to drink to much water

From the text:

"Lennie!" he said sharply. "Lennie, for God' sakes don't drink so much." Lennie continued to snort into the pool. The small man leaned over and shook him by the shoulder. "Lennie. You gonna be sick like you was last night."

"George unslung his bindle and dropped it gently on the bank. "I ain't sure it's good water," he said. "Looks kinda scummy."