Toys in the cockpit will keep her where you want her. Stop up the cockpit drains and add 12" water and you have a great pool. Install a clinometer and show her where 10 degrees is. She's not allowed out of the cockpit if the black ball is left or right of 10. Works. Having mom occupy her with books works. Use a tether and harness, IMO as important as the life jacket right now. Show her what a white cap is; if whitecaps, not allowed out of the cockpit. Explaining stuff to kids, even young ones, works. Our daughter always listened until she hit 14 years... But that's a different issue.
Our daughter has been on the water since she was 3 months old. She's 18 years now.... And starting to listen again.