A lot of DARPA's interesting marine projects relate to anti-submarine warfare, like this pilotless ship I talked about last year. Now they're looking for an underwater sensor array. I've done some quick CAD of what this might look like, as seen in the photo above. Underwater arrays are nothing new. They can look like buoys, capsules floating underneath the surface, they can even be disguised in something that looks and floats like kelp.
Here DARPA has in mind something that would be in deep ocean, at or near the bottom. This brings to mind my rough idea for a series of pressure resisting domes you could drop to the ocean floor. Sensor arrays are kind of a neat thing to begin with, a bunch of nodes communicating with each other and sharing information. So its a two fold problem of designing something that's mechanically hardy in not only high pressure environments but also cold, corrosion and have to be long lasting or incorporate some clever mechanism to surface itself and be easy to replace.
Working in the ocean is not unlike working in space; you can send your hardware into it but if something fails it's best to have as many possible methods of fixing it from a distance as possible. Meaning communication between nodes and to your surface point would not be trivial. And by the way the FBO is written you can tell DARPA's not looking for amazing innovation, they're just looking for something that works. Seems like a good opportunity for a small interdisciplinary unit to make something effective.