You can use a 12v fan but expect it fail fairly quickly due the fact that it will actually be running at 14+ Volts most of the time due to the alternator running while it is running.  To add some more details to what Alex posted, here is a link to a previous post that had a lot of good information on this subject:  https://amelyachtowners.groups.io/g/main/message/34465

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Mark McGovern
SM #440 Cara
Deale, MD USA