Can you get lockjaw from being exposed to, or drinking sewage water?

Doubtful.: Tetanus from bacteria growing in the bowel is rare after the age of 1 year. Other diseases (too numerous to count) are more likely. Tetanus is possible with a puncture wound, though.

Exposed - yes: Lock jaw is a result of a tetanus infection, classically seen when the skin integrity is broken by a cut or puncture, imbedding the tetanus bacteria into the wound. I have never heard of a case of lockjaw from drinking contaminated water, but you could get other serious bacterial infections!