The really good big fat yellow rope made for tow surfing would be best–but those are super expensive.
I’ve settled for a rope that is made for towing inflatables behind a boat. You’ll have to adjust the length. It’s made of polypropylene, so it floats–it has a thicker diameter than a ski rope so I like it better.
Towing perpendicular to the waves (straight out) will always do the nasty slack then tight. Avoid towing straight out in waves, when possible. And for sure don’t try to tow up on foil straight out through waves.