The surfboard has what appears to be jets on the back. Fins are attached to each of the jets.
It’s not exactly clear what this surfboard does or what the tech added to it is for. However, given the existence of hoverboards, we can assume that this is some kind of hover surfboard. Perhaps the jets allow it to fly higher and/or faster. It’s also possible that, like surfboards of our 2015, it’s only for use on water and the jets allow it to be self-propelled.
Given that there’s no indication that Hill Valley is near the ocean, the flying surfboard seems to be the more likely scenario envisioned by the movie creators. In either case, it’s not technology that exists in our 2015.
Prediction Accuracy: Failed