t would be crappy adventure design if the game system didn't offer such abilities like the reality inside Ocean's 11.
Since the game engine does include those abilities specifically so the inhabitants of the world can use them to overcome challenges and the game world and adventure should include varied challenges, then it isn't crappy design.
In nonmagical stories, there are still many things the protagonists can't do that other specialists could do. If one of the
Ocean sequels threw at our heroes the challenge of breaking into the International Space Station, well, nobody on either the boys' team or the girls' team has access to a spacecraft or the skill to fly one. In a different movie starring astronauts that part of the plot might be trivial, but in this movie there's going to have to be a subplot where they find a clever way to steal or stowaway or con their way onto a rocket.
(Incidentally, I would watch the
hell out of this. Get on it, Hollywood.)
But if the writers were like, "Well, nobody on the crew is an astronaut, so end of story, go home", that would be a crappy movie. And if a DM is like, "Well, none of you can cast the
water breathing spell, so end of adventure, go home", that is crappy adventure design. It remains crappy adventure design irrespective of the fact that the game system offers this ability in principle, just as our
Ocean movie remains a crappy movie irrespective of the fact that astronauts do exist in the world.
Forget about nonmagical characters: there's no guarantee that the party mage knows the spell in question either. Like I said earlier, Gandalf pretty obviously doesn't know
fly. And it's not even just about spells. In my early days of D&D, and in games based on classic D&D tropes like
Baldur's Gate, my number one priority was not making sure there was a mage in the party -- it was making sure there was a
rogue in the party. Because locks and traps were far more common obstacles than anything that required a specific spell to get through. And I
hated it. Exactly the same logic applies here: if there's a locked door and the DM says, "Well, none of you is a rogue, so end of adventure, go home", that is crappy adventure design. A challenge that can only be overcome by a specific character build which may not be in the party isn't a "varied challenge". It's the opposite. It's a narrow, railroaded challenge, and DMs should avoid presenting such challenges at all costs.