Opinions On SinSere, I, myself am alright with it, but your a bit more experienced on the subject of Sinbad, I just see them as parallels who found a sort of equal in each other.
I don't know the ship names, so I'm assuming this is Sinbad x Serendine.
I like your use of the words "parallel" and "equals." Seren is one of the few people that actually stood close to the same level as Sinbad (for a short time), and she didn't bow to his choices like his household members. I think it would have been really good for Sinbad to actually have a rival to keep him from gaining a monopoly. She was also one of the few people that made Sinbad recognize his own weakness and lack of planning. At the same time, Sinbad was very important for Serendine to recognize her own lack abilities. They played off of each other really well as foils for each other.
I don't see any romantic attraction between them in canon, but I do see the usage of a lot of tropes that are common for the starts of romance series, so it's possible that things could have gone that way some time down the line. I do think they would have grown a tight bond if it weren't for a lot of things pitting them against each other. Sinbad was very hurt by her death after all. I understand where the ship came from :3
I will say that when I think about a potential ship between them, they both end up embodying "I can make them worse." Their dynamic would be really interesting as a villain couple, or high drama.
For full disclosure, I did ship them for a while, and really liked Serendine's character. She's exactly the type of character I would have projected onto when I was young X3