Sometimes the water line is below the transom. Have you had it in the water to see? Most likely with someone in the cockpit it should be there at least.

Additionally, if you measure the distance between the clamp inside and the anti ventilation plate on a regular shaft 15" motor you might find it to be 17 or 18 inches which would be about right. On my 2.5 horse Suzuki it is 18" which is typical. That would put the anti ventilation plate sufficiently under water to be effective across a decent range, though perhaps not all ranges, of water.

I've had to zig zag across bigger, short waves to help keep the prop in the water but only on rare occasions.

Sounds like 15" will work if the bottom of the transom is at or near the water line.