Underwater acoustic communications channels are complex and rapidly time-varying. These channels fall under the category of doubly-spread fading 
channels. We will present both channel models and recent sea-trial 
measurements that illustrate the nature of this channel type and confirm
our modeling approaches.
We then present progress towards a communications signaling format that is tailored to the challenging dynamics the channel typically presents. This system is based on frequency-domain signaling and iterative demodulation/detection and estimation. The design of this system and initial performance results will be discussed, and still open questions and their proposed solution approaches are examined.