Pathfinder is a naval engineer. Similar to Freakers on land, these are used to assist T2 constructors and to quickly build T1 units in the field. Notably, they can build Floating Nano Turrets, plant powerful naval mines and produce T1 combat units such as Enforcers, and Supporters.
Never underestimate the destruction a carefully positioned minefield can bring about. With them you can rapidly secure large areas. Mines are effective until you are able to produce the more powerful T2 ships to fend off hovercraft assaults.