Today, I have built a U-boat, a German submarine from World War 2. 
During the war, 218 allied warships and over 650 were sunk or damaged by German submarines.
They were used effectively in unrestricted submarine warfare, in which German submarines were ordered to attack any allied vessels they could. 
Type VII Statistics:

Displacement:	769 tonnes (757 long tons) surfaced 871 t (857 long tons) submerged[2]
Length:	67.1 m (220 ft 2 in) o/a[1] 50.5 m (165 ft 8 in) pressure hull[1]
Beam:	6.2 m (20 ft 4 in) (o/a)[1] 4.7 m (15 ft 5 in) (pressure hull)[1]
Height:	9.60 m (31 ft 6 in)[1]
Draft:	4.74 m (15 ft 7 in)[1]
Propulsion:	2 × supercharged 6-cylinder 4-strokediesel engines totalling 2,800–3,200 PS (2,800–3,200 shp; 2,100–2,400 kW). Max rpm: 470-490[1]
Speed:	17.7 knots (32.8 km/h; 20.4 mph) surfaced[1] 7.6 knots (14.1 km/h; 8.7 mph) submerged[1]
Range:	8,500 nmi (15,700 km; 9,800 mi) at 10 knots (19 km/h; 12 mph) surfaced[1] 80 nmi (150 km; 92 mi) at 4 knots (7.4 km/h; 4.6 mph) submerged[1]
Test depth:	230 m (750 ft)[1] Calculated crush depth: 250–295 m (820–968 ft)[1]
Complement:	44-52 officers & ratings[1]
Armament:	5 × 53.3 cm (21 in) torpedo tubes (4 bow, 1 stern)[1] 14 × torpedoes or 26 TMA or 39 TMB mines 1 × 8.8 cm SK C/35 naval gun[3] with 220 rounds Various antiaircraft weaponry