...I would like to use the Interchange block (from the Rate library in ExtendSim Pro) to represent the berth. Vessels would come to either load or unload. Can I use just one Interchange block to both load and unload?

Yes. In fact, there are three different ways to model a berth that is used both for loading and unloading:

Solution 1 -  One Interchange block simulates the ships at berth and the berth itself.

Solution 2 - One Interchange block simulates the ship loading or unloading and Tanks and Valve simulate the loading and unloading of product to and from the berth.

Solution 3 - One Interchange block is used for unloading and one Interchange block for loading.

All three solutions are illustrated within this freely downloadable model that illustrates how to load and unload at the same physical location.

Note: This model was built in ExtendSim 9 so can be viewed in either ExtendSim 9 or 10.