Even the 6-ish second thing does not work reliably if the boat is underway when you hand catch. You can test this while practicing hand catches on the ground by just walking briskly to simulate the boat and
I2's motion while you try to shut down after the catch. In my experience (dozens of commercial flights from sailing vessels, including tall ships, and associated land-based practicing), you can hold the left stick straight down for a VERY long time and the motors will keep spinning if the bird thinks it is still moving forward. For safety, and to retain the confidence/respect of the people who have hired you to film their expensive yacht or tall ship, the last thing you want to do is be fighting your bird after catching it, while the boat is moving, praying for the props to stop.
The emergency kill procedure works reliably, and is the way to go (left stick in and down to 4:30 and press -hold RTH button) .
BTW, I always use an experienced assistant, with whom I've practiced, to catch the bird for me when working from moving boats at sea. While I have practiced one hand catches on my own, while standing firmly on the ground and operating the RC with the other hand, but I don't think this is a very safe or prudent way to operate, and consider it to be a last resort emergency procedure (if the landing gear fails to lower, for example)
To avoid most of the other problems mentioned in the thread above (compass errors caused by metal hulls, radar interference, etc.) we sometimes operate from a smaller non-metal boat (zodiac/dingy) shadowing the one I'm filming. On these smaller boats using an assistant for hand release and catch is still highly recommended, and someone else pilots the boat, so there are at least 3 of us on board, which can be tight.