NavList:

As it happens, my father (Robert M. Clements) is a Latin teacher. His reply to my version of this question:

Argo (a Greek word) was absorbed into Latin as a fourth declension noun, perhaps because ship names were all fourth?  That's a guess.  But tree names (quercus, pinus, etc.) are all fourth, and ships are made of wood…  A bad guess, probably.  But it is fourth, and the genitive in the 4^th ends is ūs.  That line is a macron.  Does that help?  It would indeed make the genitive Argūs navis, of the ship Argo.    

 

So that's the typographical piece of the question, anyhow.

John Clements