If you HAVE to limit advertising to certain areas I'd hit the coastal cities of CA and FL then some of the big lake areas:
Lake Havasu (obviously)
Lake Michigan
Lake Tahoe?
Lake Cumberland
Lake Placid
CA - Bay Area, LA, SD, etc
FL - Miami/Ft Lauderdale, Tampa, Daytona, etc
TX - Houston/Galveston
NC/SC - Wilmington/Myrtle Beach
But I would definitely start the social media ring and start EZ-SKI business Twitter, Facebook and Instagram accounts with all of the information in your usual ads and try and go as national as you can. I'm sure there's people in Alaska or Maine that could be in the market for one.