Advanced Construction Sub is the T2 sea constructor. Their typical first tasks are  upgrading your metal extractors, then building fusion reactors and advanced metal makers.

They have a higher build power than the T1 Construction Ship, and boost your metal and energy storage by 150. Guard them with T1 constructors or naval engineers to assist with building.

In addition to T2 economy structures, they are able to build advanced torpedo launchers and the underwater experimental gantry.