I spent a day tow foiling with Spoiled by the Foil in San Diaago. The boat captain said that when the water was less than 10-12 feet the boat could NOT kick up a wake big enough to efficiently foil behind. The bottom was messing with the wave energy.
I figured they had plenty of experience, aka 1,000’s of hours behind a boat with all the high tech gear, depth gauges, wash plates . . .
So we need 10-12 feet or more. This is consistent with what I have experienced on our downwind run Coyote Pt to 3rd Avenue in San Francisco. At the end of the run the bumps disappear. When you get off it’s only 5 feet deep. NO GOOD!