The Old days.
The first protection for underwater port attacks date from World War II.German U-boats slaughtered merchants in the first fase of the war. At a certain time they where able to get real close to allied harbors (up to New York) that they proposed a real threat for docking ships. In most places a simple net was put between harbor entrance sites to prevent torpedo's entering the ports. The simple construction worked well for a while but when
the tide turned in favor of the allies, axis forces had to deal with the same problem, they also put up nets an mines however, new weapons where able to avoid the net. In 1944 some Dam's where destroyed by "bouncing" bombs dropped from Lancaster bombers. These so called Dam busters where able to destroy some dams, flooding several area's in Germany. The Bombs where not "smart", they where just fast turning barrels, filled with explosives. They where dropped, spinning very fast, bounced of the water and rolled down at the dam, exploding with a depth delay way below surface.