For reference: this should give an idea of the rate of progress of submarine design during this period.
British 'C' class submarine, designed 1906:
287 tons displacement, speed 13 knots (surfaced), range 1,000 nautical miles, 2 torpedo tubes with 4 x 18" torpedoes carried.
British 'E' class submarine, designed 1911: (the main type in use during the first half of the war)
655 tons displacement, speed 15 knots (surfaced), range 3,000 nautical miles, 4 torpedo tubes with 8 x 18" torpedoes carried, 1 12-pounder deck gun.
British 'L-50' class submarine, designed 1917:
960 tons displacement, speed 17 knots (surfaced), range 4,500 nautical miles, 6 torpedo tubes with 12 x 21" torpedoes carried, 2 4" deck guns.
German submarines were very similar:
The U-19 launched in October 1912 was the same size as a British E-class (650 tons), similar in speed (15.4 knots), much longer ranged (7,600 n.miles), and carried 6 x 19.7" torpedoes and an 88mm gun.
The U-93 launched in December 1916 was slightly smaller than a British L-50 class (838 tons), the same speed, again much longer ranged (8,300 n.miles), and carried 12 x 19.7" torpedoes plus a 4" (105mm) and 88mm gun.
French and Italian submarine design in 1914 was probably one level behind British and German. Same goes for the Japanese, who were using British-designed submarines at this time. Russia was two levels behind, with a lot of one-off prototypes and botched designs plus some gems. The USA was equal to Britain and Germany. Austria-Hungary got most of its submarines from Germany after the war began. Other countries either had no submarines (or bought them from abroad) or at most were building tiny, short-ranged craft that would figure more as coastal defences than naval vessels in HoI terms.
Finally, for comparison's sake, a German WW2 U-boat (Type VIIb) was 741 tons, 17 knots, 5 torpedo tubes with 14 x 21" torpedoes, an 88mm and a 20mm deck gun. In other words, much the same as its 1917 predecessor except for a slightly better armament.