Can MSR fuel bottles be used as a water bottle? Are they safe for drinking out of?

The short answer is no. MSR fuel bottles are not considered food-safe (and haven't been designed or tested for that purpose). Therefore, you should not use MSR liquid fuel bottles for any form of water storage.

The inside of our fuel bottles is made out of raw aluminum. Petroleum based fuels have no reaction to raw aluminum, however, water and alcohol based fluids do. You will never see a raw aluminum can for food or water storage. As an example, aluminum soda cans are coated/lined - they are not in a raw state.

Additionally, we do not recommend rinsing out MSR liquid fuel bottles with water or alcohol. As an example, if you have been using white gas, empty the bottle and fill it with kerosene, (both petroleum based). Any slight remaining residual fuel would not have any impact. Washing out the interior of the fuel bottle with soap and water could pit/damage the interior of the bottle so this should be avoided.