I intend to do two more posts on the subject of the First World War- one more on its legacy and a second on peace in general.
The First World War is not as well known in the naval area as the Second. There was only a couple of large sea battles in the course of the war- Jutland being the most famous.
However, this war set a lot of elements that would make the later Battle of the Atlantic in the second war so bloody for all concerned.
Submarines had been around for a long period and used in war for about half a century. They were more submersibles at this point, spending most of their time on the surface.
Initially the Germans adopted the Prize Rules of the period (specifically from the two Hague Conventions)- challenging merchant ships, allowing the crew to get to a place of safety, which didn't count lifeboats, then sinking the ship. A British Admiral, John Fisher, worried about the Germans going to unrestricted submarine warfare- as a sub could not take the crew of a merchant ship. Churchill, then thought that a civilised nation would never do it.
Fisher was right- Germany did go unrestricted, in response to an effective Allied blockade of Germany. During this process it sunk the Lusitania- that, other attacks and the later Zimmermann telegram, brought the US into the war, leading to Germany's defeat.
For both sides, submarines were noisy, dirty and cramped. Being sunk could lead to a highly unpleasant death.
Some of the U-Boat commanders committed atrocities (others didn't) and there was considerable public hatred of them in the Allied countries.
Depth charges and sonar appeared in this war, to be perfected later.
Being on board a sinking ship, flooding rapidly, while possibly injured, must be an awful way to die. Knowing also that your family won't have a real grave to go to either. Those who served in the naval and merchant ships in both wars were truly brave men and women.