This study developed a 3D concrete printing (3DCP) system that can print not only in air but also underwater. This underwater 3DCP system is equipped with many distinct technologies, such as a technology to supply the printing material to the nozzle tip at a constant rate by detecting its amount in the printer hopper. Using the developed 3DCP system, the effect of nozzle details on underwater print quality and hardened properties was investigated. The straight-line printing performance underwater was evaluated using five nozzles: a nozzle without a trowel (Nozzel#1), a nozzle with fixed trowels attached to both sides (Nozzle#2), a nozzle with trowels attached to the back and both sides to constrain five sides (Nozzle#3), a nozzle with a three-sided trowel inclined by 30° (Nozzle#4), and a nozzle with a roof added to Nozzle#4 opening (Nozzle#5). Nozzle#4 yielded the best print quality and hardened properties. In addition, an underwater curved shape printing test was performed using Nozzle#4, the problems that occurred in this test were analyzed and solutions were suggested.