Pathfinder is a naval engineer. Similar to Twitchers on land, these are used to assist T2 constructors and to quickly build T1 units in the field. Notably, they can build Floating Construction Turrets, plant powerful naval mines and produce T1 combat units such as Oppressors, and Riptides.
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.