You are fortunate it broke off while on the hill!!!
Try to extract the remnant. A good mechanic can usually extract a broken off NPT fitting, but sometimes they are too stuck. Once out, the fitting can be replaced. Absolutely use the right alloy for the fitting. That it broke in the first place implies the wrong alloy was used. Brass in bronze, no good. Bronze in bronze, ok.
Chances are if you have a slow moving boat, there is no need for a cooling line at all. You could cap it off or plug it, and just monitor temp until packing seats in.
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