Eels are stealthy assault submarines. They are equipped with a powerful torpedo launcher that can deal lethal damage to structures and ships within seconds. However, they are also very vulnerable due to having low health. It is kind of a glass cannon. 

They are invisible to radar and to units on the surface, but are visible to sonar and other submarines.

If your enemy is not careful, you can sneak one or two of those into their base and cause havoc. 

Their main use is destroying underwater metal extractors, and trapping fleets which don’t include proper anti-submarine units, like destroyers.