Voyager is a naval engineer. Similar to Butlers 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 combat units such as Destroyers, Platypus and Dolphins.
Don’t underestimate the damage you can do with a well-placed naval minefield. Use them to quickly secure large areas from hovercrafts, particularly early after you take control of the sea. Use mines until you are able to produce the proper T2 ships to defend yourself.