A bird sits upon a mountain. If you look at the radicals, the bird is quite large and the mountain quite small. Think of this in the mnemonic as well. The reason the mountain is small is because you can only see the top of it, sticking out of the water. On it is the bird. This small mountain with the bird on it is an island.
But this island doesn't only belong to the bird. In fact, the bird shouldn't be on this island, because it belongs to She-Man (しま), He-Man's girlfriend. She-Man comes out of the water and grabs the bird, eats it, and then goes back into the sea from whence she came.
Currently there are no similar looking kanji which exist in the WaniKani learning system