Predators and water barriers

Permaculture books (Mollison, etc) tend to have drawings of happy ducks hanging out on an island in a small pond. On the shore side they also show a protected coop which is open to the water but fenced from land.

My question is really on the former - whether a water barrier is effective (or is the happy duck on an island a myth?), and if it does work what distance from shore is required?

I can see how an island is effective against some predators, but for example raccoons can swim? And what about snakes? rats? etc?

Thanks for any thoughts or practical experience

I thought about doing the same thing here: using the island in our lake for a chicken pasture and the lakeside as a goat pasture barrier. Never did it though. It's too expensive a lesson to risk it for us, and besides, once our water freezes over for the winter the predators would end up with a free pass anyway.

I imagine it could work great in some locations, particularly if the perimeter of your land is already fenced to keep predators out/escapees in.

I wonder if its possible to teach the birds to use a door 3' above the water level, with a small gangplank. Ducks are able to lift off from the surface of water better than any of their predators. The hard part will be teaching the ducks to use it. So maybe have an easier door where you can herd them in at night, but keep closed once the ducks have figured out the program.

I haven't tried this exactly. But I have noticed that teaching chickens to jump to a high door to get inside a coop is not easy.