Here's the first dip in the ocean. Yes, the boat is in a highly unfinished state, but you get the idea. I quickly screwed some plywood on top to mimic the shape and weight of the yet-to-be-installed solar panels. The water was calmer than I would have liked, but it was a great first ocean test.
Here's the data from the test. Speed data was obtained with an iPhone running a GPS tracking app and a voltage and current logger to measure power draw. I'm expecting a 24-hour average of 20 watts from the solar panels, so 3 knots looks to be a reasonably attainable speed.