The lower operating temperature for the raw water cooled thermoststs is because salt water really does scale very aggressively if it is allowed to run at the temperature common in heat-exchanged motors.... perhaps 90 degC, 194 degF.
Raw water motors really must be a bit overcooled.... they cannot be allowed to run so hot.
Cast iron is a common material. I am surprised you have zincs. They must blast away quickly, do they not?