He tell him not to drink so much or he ll get sick. George told Lennie this because the water that Lennie was drinking was covered with dirty substances. If Lennie continued to drink the water, he would more than likely get sick because it was not clean water.
This pool was before George and Lennie reached the ranch. This water was standing and it was unknown how long the dirt in the water had been there. If it had been flowing water, it would have been better and less hazardous. However, standing water is definite to make a person sick.