It depends on which outrunner you are speaking of in particular. Most seem to use a forward cooling section or structure to circulate water in these motors. Do a search for "little screamers" or "scorpian" and you will find a bunch of pictures and information.
I just make a water cooled motor mount from some 6mm aluminium stock..
Here's a pic of one. Note that the tubes had'nt been installed in the first pic.
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