Question for boat mechanic?

I have a 93 mercury 200. I have replaced the water pump and the tstat's. The water pump seems to be working great but as soon as I put the tstat's in the engine stopped pushing water out of the little pee hole and it started to overheat. I turned it off and removed the tstat's and the temp came back down to normal temp without a tstat. Do I need them? If so why would it over heat with a 143 degree tstat installed?