It's the middle film in a Sinbad trilogy. These films were celebrating the 40th anniversary of Nippon Animation and they have nothing to do with Magi: The Labyrinth of Magic, the anime TV series that aired at the same time and also involves Sinbad, Ali Baba, Aladdin, etc. They're made for different audiences. This film trilogy was clearly for younger children, with an old-fashioned animation style and nothing even remotely suggestive or dodgy. It's safe, bland children's entertainment. It's okay. I wouldn't recommend it, but I didn't mind it. I'd say the audience's target age is probably about six, or thereabouts.
The main character is Sindbad, which seems to be his name in Japan. (I think that's a closer match to the original Arabic than "Sinbad", actually.) He's on a sailing ship, but he's not its captain because the heroes of a children's film like this must themselves be children. He seems to be the ship's boy. His friends there are an even younger, more immature boy called Ali (presumably Ali Baba?) and a girl called Sara who's a magical princess.
Sindbad has no personality. Ali has a personality, but only because his story role is to be more childish than Sindbad. Sara's the only one who actually gets character-based storytelling, when she gets upset because people are asking her to do magic.
The film's driven by its plot and its fantasy concepts, really. You'd struggle to remember anything about the characters, but the film's a halfway decent children's adventure. It has our heroes getting stranded on an island that wasn't on the charts. (See the movie title.) It has blue horses with gills and fins instead of hooves. It has a truly enormous Cyclops that resembles Ray Harryhausen's in The 7th Voyage of Sinbad (1958). It has gargantuan island-fish that fill me with nitpicky questions but look awesome anyway.
It even has a villain with magic carpets, a flying horse and a steampunk submarine, although none of that stops him from being a bit rubbish. This isn't the kind of film where you expect serious villains. He's defeated by two children running up and being cheeky.
There's not a lot of point in this film, really. The story doesn't mean anything and there's not much to the characters. Instead it's just a simple, earnest runaround with deliberately retro animation and lots of Middle Eastern storytelling references. It even has Aladdin's lamp, although oddly neither Aladdin nor the genie. You won't remember this film, but you won't hate it either. It means well.