Oh dear, Brian, you had to ask
OK, maritime law is complicated and I would be the first to admit that I only have a cursory knowledge but :
1) the main legal principle affecting naval conduct was
Declaration concerning the Laws of Naval War (1909). At that time, the assumption was that all civilian ships were exempt from military action except where they maybe perceived to be carrying "contraband" (see link). You may know of the outrage over the sinking of the Lusitania and the cliam by the Germans that it was a legitimate target because it was carrying ammunition.
http://www1.umn.edu/humanrts/instree/1909b.htm
2) After WW1 the next main treaty was the
Treaty for the Limitation and Reduction of Naval Armaments (1930), and especially Part 4:
- The following are accepted as established rules of International Law:
- (1) In their action with regard to merchant ships, submarines must conform to the rules of International Law to which surface vessels are subject.
- (2) In particular, except in the case of persistent refusal to stop on being duly summoned, or of active resistance to visit or search, a warship, whether surface vessel or submarine, may not sink or render incapable of navigation a merchant vessel without having first placed passengers, crew and ship's papers in a place of safety. For this purpose the ship's boats are not regarded as a place of safety unless the safety of the passengers and crew is assured, in the existing sea and weather conditions, by the proximity of land, or the presence of another vessel which is in a position to take them on board.
http://www.microworks.net/pacific/ro...don_treaty.htm
Though the rest of the Treaty lapsed, Part 4 was reiterated in 1936, and no other treaty replaced it before the war.
So, as I said, the conventions were that civilian ships were not to be attacked. However, they could be searched for contraband (material that could help the war effort), and then taken into possession or sunk. Now, that obviously cannot be the case for aircraft i.e. searched, so the belligerents erred on the side that they carried contraband and shot them down.
However, read the bits about neautrality and the travel to and from a neutral country. It seems that, again, the principle that those travelling from or to a neautral country were "protected", hence why there were scheduled flights between Portugal and Britain (and generally unmolested)
I shall stop rambling now
A