Why does Nadia rudely turn down Jean's attempt to compliment her in episode 20? Awkward though he may have been, he was trying to be nice and he didn't deserve to be yelled at.
She's jealous of Electra; Grandis had told her that she may be in danger of losing Jean to her. Unfortunately, Nadia is not very good at expressing her problems in an appropriate manner. She has never learned how to convey herself through anything except rage. So instead of talking to Jean about her concern calmly, she instead shouts at him "you're always looking for some silly excuse to visit that woman, aren't you?! You're a cretin!" She expects Jean to read her mind instead of being very clear about what she wants from him. Jean, of course, is not capable of doing that, which causes confusion from him. Luckily, Nadia does learn to be more forward eventually to get what she wants from him.
Why does the action become jarringly cartoonish during the island arc? What's up with all the Looney Tunes stunts (Jean stepping off a cliff in episode 26 and falling, Wile E. Coyote style to the ground, swallowing water in the captain's pod and swelling up like a balloon, etc.)?
The answer? Hideaki Anno was not in the director's chair at that point. Up until episode 22, Nadia: The Secret of Blue Water was a genuinely epic, sometimes dark sci-fi adventure story with the occasional bit of comedy. Even some of the funnier bits in the first 22 episodes weren't as drastically "cartoony." Because the show was given a last minute extension by the executives at NHK, Gainax didn't have the time or money to do these extra episodes properly, hence why they were outsourced to other studios in Japan and Korea. But the real problem was either that Shinji Higuchi apparently didn't share the same vision that Anno did about the characters or the physics of Nadia prior to stepping in starting at episode 23, or maybe because the scriptwriters perhaps had intended for it to be a cartoonish story? Remember that Anno disliked the original screenplay for Nadia and often changed it to suit his standards for episodes 1-22 (As referenced in http://www.mangauk.com/post.php?p=the-world-of-hideaki-anno ). Perhaps Higuchi didn't have the gall to do so?
According to the Japanese sources, GAINAX accepted NHK's prestige order to film Jules Verne's '20,000 Leagues Under the Sea' and 'The Mysterious Island' in a 39-episode series. However, GAINAX only had the budget for 26 episodes, so the general director decided to let the animations for the middle part be drawn in Korea. Since the drawings did not meet the quality requirements, they were revised again in Japan in individual episodes. Due to the poor quality, episode 34 was almost completely reworked in Japan, at the director general's own expense. (See:http://blog.livedoor.jp/thx_2005/archives/51803823.html)
The whole Africa arc. Episodes 32 and 33. That whole storyline just doesn't fit with the atmosphere of Nadia, and comes at precisely the wrong time in the series, especially where the character relationship dynamics are concerned. Nadia just admitted Jean was more important to her than the Blue Water. She embraced him nude. And there was no conflict between them while they were riding to Africa on the Gratan (except for Nadia being somewhat afraid to tell Jean about what happened while she was in Red Noah's control chamber, and even so very respectfully if quietly). So why the hell does she get hearts in her eyes (literally!) with some stranger in an African village who does little more than just recognize her Blue Water and vaguely implies he knows about Tartessos? And treat Jean like trash during that same sequence when he's done at this point absolutely nothing to deserve it? It doesn't make any sense, plus it is both out of step and seriously detrimental to Nadia's character.
While one could make the arguments that Nadia is simply afraid of telling Jean who she is and so feels she has to bury her love for him and re-direct it toward someone who she doesn't know to fear rejection by the former, the whole thing is handled much too suddenly and with zero explanation. There is no build-up to it. Even the friendship between Jean and Electra didn't so quickly escalate to such extremes that fast (nor did it ever). Plus, there are other warning signs in these African episodes that the whole thing feels rushed. For instance, the villager claims to be roughly thirteen years old, but he looks obviously like he's 17. The plot elements are cartoonish as well: A village that worships a goddess lion with big testicles? Whose treasures are just a can of food and a magic potion that can make anyone run super fast? The genuinely awful animation? The characters all not behaving like themselves? (Grandis even falls again, temporarily, for her Snidely Whiplash-like ex-fiance!) Yeah, it's not just Nadia's sudden flip in character that's problematic (although it is the worst issue of that sequence). The whole thing feels very off from the start. (This, in a show where we were previously treated to wonders beneath the ocean, murders, mystery, undersea battles, even death! There was a certain grittiness to the canonical episodes that is absolutely missing from this side story as well as the island episodes.) The whole two-episode affair (plus the subsequent singing episode) is also forgotten in episode 35, never once talked about again. Even the vaguely referenced "silver city" by the African villager is never shown. Nor even mentioned during anywhere else in the show. So, yeah, something was obviously fishy with the whole "affair".
For the japanese Audience or the attentive viewer it is not an headdcratcher, because of some factors:
In the preview to episode 31 Nadia says she's afraid Jean will hate her if he finds out her secret heritage. Unfortunately, the previews were only aired on Japanese TV.
Sanson gived Jean in Epsiode 28 the hint, that Nadia is a girl who likes physically attractive boys. Sanson seems right when Nadia first met a physically attractive boy in her age. Nadia shows normal behavior for a pubescent girl. Interestingly, Jean acted the same way in Episode 1.
Grandis says in Episode 32 that if she falls in love with a stranger at first sight, Nadia is now an adult. This means that Nadia has evolved by shedding her basic distrust of strangers. Within the Arc, Nadia and Jean learn about the downside of being in love. The arc ends with both learning to express their feelings towards each other.
Gainax's Toshio Okada explains the situation behind the making of Nadia. ("Conscience of the Otaking" interview)
"Nadia was true chaos, good chaos and bad chaos! **laughs** On Nadia, Anno didn’t direct the middle episodes, Shinji Higuchi did. And some episodes were directed in Korea–why, no one knows exactly. **laughs** That’s real chaos, not good! What I mean to say is, controlled chaos–that’s good. Controlled chaos is where you’ve got all the staff in the same room, looking at each other. But on Nadia you had Higuchi saying, “Oh, I’ll surprise Anno”, hide, and change the screenplay! Screenplays and storyboards got changed when people went home, and the next morning, if no one could find the original, I authorized them to go ahead with the changes. No one can be a real director or a real scriptwriter in such a chaos situation. But on Gunbuster, that chaos was controlled, because we were all friends, and all working in the same place. But on Nadia, half our staff was Korean, living overseas. We never met them. No control."
Series proper
How did Nadia survive the doom of Tartessos, while thousands died, and being transplanted to Europe to work in a circus? And how did she ends holding the Blue Water, the most precious artifact of her native civilization, for whom wars were made and thousands died? There's no explaination of these in the series nor the omake.
In the Japanese and German versions, Emperor Neo claims that he rescued Nadia from Tartessos.
There's no explaination of why King was able to adopt human manierisms, even outside the filler: for exemple, he was able to outsmart Marie in classroom.
It seems he was originally set to be revealed, in the ending, of extraterrestrial extraction but these plans were shelved.
Was Nadia's ability to speak to animals personally bound to her or the Blue Water? Was she able to kep talking to animals even after the Blue Water went as dead as lead? For the mater, was she able to read Atlantean for the same reasons?
Did Nemo forgot about the existence of the ship which was set to become the N-Nautilus? It would have spared him and his crew a lot of work.
). Perhaps Higuchi didn't have the gall to do so?