Hulk is a heavy and durable sea transport. 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 armor is able to withstand a lot of damage, so it is relatively safe to transport your Commander this way. When they destroyed, all units inside go down with them as well.