Nadia's story is the strong point. It's definitely one of the best stories I've ever seen in a series, up to a point. You may or may not know already that it loosely follows the plot of Jules Verne's 20,000 Leagues Under the Sea, but it's mostly an original work. I personally consider it to be part of a trilogy along with Gunbuster and Evangelion because they're all very similar series, Nadia is of course the most obscure of the bunch but each of them share some of their themes and it's noticeable after seeing them all.
Everything up to them is fantastic, but the two story arcs that "killed" the show are one set on a remote island that feels kind of like some fever dream version of Gilligan's Island with three kids and a baby lion. Most of the interesting characters totally absent here and the events leading up to this are mostly ignored. The episodes are pretty confusing and shockingly dumb. This arc is immediately followed by some inexcusably messy venture into some African village that thinks a can of food is a pillar of silver gifted to them by God or something. But it gets back to kicking ass in just the final 4 or 5 episodes, thankfully.
The animation is mostly well done on its low budget but some episodes certainly suffer a lot, most of the details are typically conveyed in a very minimal fashion and whenever there is an error it's especially difficult to look at. One thing I couldn't get out of my mind even after I finished watching the show is a part where a character's entire forearm is see-through. Chilling.
There are also lots of good characters in the show. The Nautilus crew is well fleshed out and the Gratan team is probably Gainax's first major endeavor into GAR characters. The Neo-Atlantis bad guy Gargoyle is really simple in terms of motives and personality but he's still kind of cool. The eponymous Nadia and Jean (the kid that orbits her) are unfortunately not among the good characters in the show. Nadia's personality and characterization, while you eventually come to understand it, is way too erratic. Though her boyfriend isn't too bad in terms of personality, Jean's knack for inventing is kind of abused as an unrealistic deus-ex-machina that just leaves you wondering if it's really possible for a 15 year old in the year 1889 to invent so many miracle machines. He makes like 20 planes across the series and even a functioning shower on a remote island composed of scraps. Not very plausible.
The last good thing I can say about Nadia is that almost all of the soundtracks are bangers and they're very memorable, but I don't really know what else to say about that.
Overall, if you are a fan of Gainax and Anno, or you're interested in something with a fantastic story and don't mind it having very few other merits, definitely go ahead and check it out. Otherwise maybe put it on hold or stay away from it.
