The daughter of Where's the Ring seized command at the top of the stretch and stayed on well in the final eighth, defeating Sans Souci by 2 1/4 lengths.
Van Lear Rose finished third, but was moved up to second when stewards disqualified Sans Souci from second and placed her fourth for interfering with Koonunga Hill, who was placed third.
David said his filly broke well, but after several rivals on the insidedashed to the lead, he opted to settle his filly off the pace.
"They ran away from her. She was leaning outward. She seemed to be happy out there. So, I left her be and asked her the last eighth of a mile," said David. "She's got gears."
The Shady Well is David's first stakes triumph since he guided Shipman to victory in the $30,000 Thoroughbred Niagara Cup Stakes at Fort Erie on October 1, 2006.
Cawaja Beach was David's first Woodbine Stakes victory since 1996, when he took the La Voyageuse with Franssica d'Amour.
"It has been a while," said David. "It's always lovely to be inside this (winner's circle) horseshoe."
The two-year-old filly is owned by Hard Eight Stables and Ace Racing and is trained by Scott Fairlie.
Fairlie said the Ontario-bred filly has always impressed him.
"She won her last race very easy," said Fairlie, referring to a 2 1/4-length score in her June 14 debut. "She made the front and her ears went up and she kind of lost interest."
Now 2-for-2, Cawaja Beach returned $5.80, $3.70, $2.50, combining with Van Lear Rose ($11.80, $6) for a $78.30 exactor. Koonunga Hill ($3.40) rounded out a $375 triactor. Sans Souci completed a $1 superfecta worth $1,520.30.
Spectacle and Showboater were scratched.