Mike drinks a lot of the water. He put the canteen back into his pack once he finished.
He felt a scratchy feeling as his skin started to turn green not even five minutes afterdrinking the water, followed soon by a forked tongue and claws. The fifteen minute his tail starts to press out and grow thick and heavy. At the twenty minute mark his head shifts and becomes much like a lizards while staying humanoid, while for the next ten minutes his body bulks up and grows larger, straining and ripping his clothes apart.
He could see himself in the reflection of the lake now Mike was 7'2" tall with 500 pounds of extra muscle.
Mike smiled, "That'ssss right, now what sssshould I do now?"