I know a guy who used to do that in a Stearman.
I wouldn't imagine it would be much different than doing a wheel landing on grass. Moving forward with any significant speed you might as well be on grass. After all, you can barefoot at roughly 30mph, right?
Figure they're doing 90kts to maintain stability, AT-6's/SNJs are trucks at any speed, moreso low speeds. At that speed...wouldn't think it would be as high a difficulty or "death defying" as most think. Even if the engine died, you'd be able to pull the airplane off the water and have a fighting chance at survival by using normal ditching procedures.