This looks like it has a lot of potential. If you had a foil that self-regulates its depth then you have the resistance of the line to the foil through the water. The rest is the air resistance of your body as you can be above the water with a line from your harness to the foil. No board needed. You just fly the kite as normal.
Its obviously going to be dangerous to have a foil flying around if it comes out of the water so you have to get it working and then keep away from everyone else, but it should be able to run fast in bad chop.
Now that looks fun and I love the first clip with the pilotless kite as it is thinking outside the box but I do think there is more potential for disaster than tethered flight with that second clip. The thing you need with speed is razor sharp control and reaction and it shown neither, not criticizing the principal of it, there could be something in it. I'm not convinced there will be boards as we know them going much faster than they do now, it will have to be something different to a board I reckon.