Marine engine need enough temperature to eliminate condensation build up, no way would I ever agree to running an engine without a Tstat. I have many years of hands on experience seeing the internal damage caused by cold temps. Griz brought up the same issue, he ran a closed cooling with a 180 Tstat. I ran closed cooling and a 180 in my 383 no way would I risk my investment. If anyone removes the oil fill cap and finds any trace of white muck they are running to cold!
M.O.P.
No matter what your beliefs are "GOD BLESS AMERICA"
Fully retired marine tech near 60 years in the biz.