Sufficient Space:  The minimum race course is 100m long by 20m wide, and must have sufficient area for run-up (at least 20-30m).  Don't forget room on shore for spectators and judges.

Water Depth:  You'll need at least a 2m depth throughout the race course and run-up if you're expecting hydrofoils to participate.  Regular propeller-driven boats need less, and paddleboats can run in even shallower water.  The course should also be free of weeds, rocks, and other obstructions.

Flat water:  The calmer the water, the better your results will be.  Protection from wind, motorboat wakes and currents is desirable.

Approval:  You must be certain to obtain the proper permission and/or permits from all appropriate authorities, depending upon the ownership of the lake (e.g., private, municipal, city, county, state park, etc.). This typically requires some form of insurance indemnification certificate to be presented to the various parties involved. Also, some sort of waiver is typically required of all race participants. A sample of such a waiver used by a club in Washington State is provided here as a reference.

Safety:  Be prepared to provide first aid and/or call for emergency services.  You'll need safety boats along the course and you may even want to arrange for on-site emergency vehicles.  Better safe than sorry!
A sample of a safety plan used by a club in Washington State is provided here as a reference.

Facilities:  Depending on the number of boats and spectators you're expecting, there should be ample parking, restrooms, amenities, etc.