As an aero-hydrodynamicist, I was involved in several aborted speed sailing projects, but still gets concerned in that topics area.

About the best locations, I formerly visited Ivanpah Lake ( border California / Nevada )for ground and French Camargue's Port-Saint-Louis of Rhône canal for water ( where LONGSHOT hit 43 kt ! ). But, after several discussions with sailors and architects, it quicly came obvious that shallow waters pounds or beaches seemed to be really the best racing liquid tracks !

Among them, "board's spots" like Tarifa ( Gibraltar ) or Fuerteventtura ( Canarias ) are undoubtly the best providers for strongs winds ( Force 6-7 ) and flat waters. The french pounds Etang de Thau ( Sète ) and Etang de Berre are exceptionnal too...

At least, it mainly depends on the boat speed / wind speed ratio you're aiming at ( high or low transverse fineness ), but flat waters are crucial whatever...

GOOD LUCK...