Recently went down a Wikipedia rabbit hole and was shocked to discover just how prevalent these beliefs were amongst the lower ranks. The German naval mutinies in 1918, the Potemkin mutiny (as well as the fact that the Tsarist navy went over almost entirely to the Bolsheviks in 1917), and the Black Sea mutiny of French sailors in the Black Sea (1918) come to mind.
I can speak to the Germany navy of 1918 best so I will stick to them.
It's a little inaccurate to describe the German navy as a bastion of leftwing activity. FL Carsten states that "a minority among the sailors" held left-wing views [1]. This minority, however, became very persuasive when considering both the social conditions of the navy and the events of World War I.
The German navy was highly stratified with officers getting the privileges of executive rank [2][3]. Engineers and lower tier sailors, however, were kept as a distinctly lower social class and their wives were not even allowed to fraternize with officer's wives [4]. This bred a great deal of contempt in the pre war navy that was only tempered with the start of the war [5].
When the war continued to drag on, the German navy was seen as a drag as it did not achieve measurable success. Particularly after Jutland, the surface fleet did not successfully engage the British despite their many attempts to strike the Royal Navy [6]. This led to the surface fleet being seen in a negative light by the larger public.
This was exacerbated by the von Hindenburg and Ludendorff dictatorship, which enacted policies that would ultimately hurt German food production and distribution, contributing a great deal to famine [7].
Together these factors led to the German mutiny, which was certainly a left-leaning movement, but again not the result of a left-wing bastion.
[1]F. L. Carsten, War against War: British and German Radical Movements in the First World War (Berkeley: University of California Press, 1982), 115.
[2]Holger Herwig, “Luxury” Fleet: (RLE The First World War): The Imperial German Navy 1888-1918 (Routledge, 2014), 111-143.
[3]F. L. Carsten, War against War: British and German Radical Movements in the First World War (Berkeley: University of California Press, 1982), 112-124.
[4]Holger Herwig, “Luxury” Fleet: (RLE The First World War): The Imperial German Navy 1888-1918 (Routledge, 2014), 111-143.
[5]F. L. Carsten, War against War: British and German Radical Movements in the First World War (Berkeley: University of California Press, 1982), 112-124.
[6]Marcus Faulkner, The Great War at Sea: A Naval Atlas 1914-1919 (Annapolis: Naval Institute Press, 2015).
[7]Alexander Watson, Ring of Steel: Germany and Austria-Hungary in World War I (New York: Basic Books, 2014), http://public.ebookcentral.proquest.com/choice/publicfullrecord.aspx?p=1681910.