He's not the only one. There have been plenty of others too, around the World, and of all three species-Gorilla, Chimp, Orangutan-both adults and young. At least three adult male Gorillas. Several juvenile Orangutans. More than one Chimp mother has drowned trying to rescue a baby that had fallen into the water. Prague recently nearly lost a young Gorilla that fell into the water- the group just sat and watched but the keeper managed to jump in and save it just in time.
Some zoos of course have not had any accidents. But Apes that have not previously been kept with water barriers (such as at Twycross) are obviously at greater risk still. The plan definately shows islands and what appear to be water moats.
Apart from the danger factor, these moats, because of their necessary width, mean the actual display areas for the animals is often small and use up large areas the Apes could have instead if it was dry land. Although I don't like other aspects of the Twycross Ape enclosures overall, I think the glass viewing walls and dry ditch system is actually a good one, both for viewing and the Apes can use the ditch areas too.
The Gorilla was more recent, it is somewhere on here too.