I think you should use Siphonophores for the cover (I saw them on the childrenâs tv show, Octonauts). They are an interesting sea creature that are a compound of many integrated zooids. Each zooid has a high degree of specialization. They kind of act like a neural network. There is a âbrainâ zooid that all of the member zooids provide information to. Here is a paper that did some research.
http://papers.cumincad.org/data/works/att/caadria2013_030.content.pdf
Here is a link to the episode.
I think we can use JellyFish for the cover. This beautiful sea creature have so many tentacles to represent numerous of new features in iOS 11
What about Sea Lions
Dolphins are awesome! Observations indicate that dolphins undergo daily cycles of activity, which include feeding, socializing, traveling, and resting. Social behavior comprises a major portion of bottlenose dolphinsâ daily activities. 
Electric eels grouped to form the number 11.
Theyâll represent the ever changing and never 'staticâ platform,
the âcurrentâ push toward AR and VR,
the âpowerâ of ML,
and the âsparkâ for new and creative apps using the âelectricâ changes in iOS 11.
âŠand now Iâve run out of terrible puns. I know, itâs âshockingâ 
The cover of iOS 11 Tutorial should be a school of 11 âTiger of Seaâ - The great Barracuda.
The excellent and efficient hunter, sleek and speedy Barracuda represents the excellent brand new features of iOS 11 that provide effective and adaptive focuses on artificial intelligence, machine learning, advanced VR/AR, powerful multitasking and much moreâŠ!
What about 11 piranhas that always hunt in large groups. Will you be brave enough to shower after you see an AR game in which eleven piranhas appear in your own bath?? !! 
They can be 11 friendly piranhas if you donât want it to be too scary. With 3D effect on the cover.
Mantis shrimp, because itâs the killer of the sea ! It owns two hammers and uses them with such a force and speed that water boil around. It can breaks clams or even aquariums with it !
(More seriously I like the ideas about Dolphins and Exocoetidae, but they were already suggested 
)
What about seashell of this type? With eleven segments!
http://proxy11.media.online.ua/uol/r2-116826ddea/5175cd9779b0b.jpg
I like the way you are thinking⊠:]
White Anemone
Whatâs this creature?
This sea creature inhabits the worldâs oceans run the full spectrum of colorsâoften in intricate patterns, advanced camouflage, and varying degrees of transparency.
Reason for choosing this creature:
I find that ios11 comes with many hard to understand topics but yet theyâre so beautiful and the Ray wenderlich team is going to bring clarity to the complexity.
Flasher Wrasse, newly discovered fish.
White Anemone and Sea Star, the creatures that inhabit the worldâs oceans run the full spectrum of colorsâoften in intricate patterns, advanced camouflage, and varying degrees of transparency. It would be great to showcase versatility and new patterns of iOS 11
11 killer whales, the most intelligent animal in the sea.
I propose âGlass Squidâ as the sea creature to be on the cover of âiOS 11 by Tutorialsâ. iOS 11 has brought in a lot of design changes throughout the UI of the iPhone & iPad and mostly it presents design queues in translucency. âGlass Squidâ family of species has a transparent body structure that blends with the background.
Crabs!! They run in schools, so why not.
I personally fall in love with a cute tiny creature often called Blue Angel (Glaucus Atlanticus). The image below is something that potentially could become a nice book cover.
WoW, I thought it was a kind of Pokemon when I saw the picture, then I realized it really exists !
Wonderful 
If you do not mind the school of fish, what about aquatic dinosaurs?
Because iOS 11 has stepped into the unreality world called AR / VR, it is not bad to feature dinosaurs that do not exist anymore.
I would like to propose the Plesiosauria.
i.e.
