Envoy is a heavy and durable sea transporter. Each one has a transport capacity of 40, with each unit carried taking up space: 4 spaces for a large unit, 2 for a medium unit, and 1 for a small unit. They are mainly used to quickly transport the Commander and land constructors across water. The Commander can be loaded or unloaded from underwater.
Their armour is able to withstand a lot of damage, therefore it is pretty safe to move Commander this way. Though when the transporter gets destroyed all units inside go down with it as well. It can unload and pick up the Commander from underwater.