Submarines can be devastating to a sea-based power. Corvettes and-Ground Attack Planes dedicated to protecting shipyards and docks and sea mines may seem like a needless expense until these facilities start blowing up in your back yard. Of course, a sea mine field is also a good investment. Mine fields are always useful as a protection. The best use of minefields is in front of a position that can be protected by infantry and scouts to take care of surveyors and minelayers.
Infiltrators are always useful.
Even if they just keep an enemy from using an eco-sphere for a few turns, their contribution can be invaluable. Remember that Armored Personnel Carriers are amphibious and virtually invisible unless on land. The only real failing of the infiltrator is that its ability to take over or disable enemy units contains the game's sole concession to probabilities. There is always the chance of failure, and the chance that failure will bring discovery and disaster. The right upgrade to units can make all the difference in a long-term game and in some of the tactical scenarios we have provided with the game.