How was submarine warfare viewed by most nations?

In the early days of submarines, they were frowned on by traditional Navies as being like snipers - sneaky, low, and completely without honor.

That vision went away rapidly though when the Germans showed just how much of a strategic weapon submarines could be. In fact, in the Treaty of Versailles which ended WWI, the Allies stipulated that Germany relinquish all of her submarines - in large part so that Allied scientists and engineers could study their technology and designs.