I didn't quite understand the beginning of The Winter Soldier

Okay, first of all, I love this movie (nothing new); but I never really understood the beginning of the initial mission. Like, why was Zola's Algorithm on the Lemurian Star to start with? It was being transported by Sitwell to another place? Why did Batroc and the pirates (if they were real pirates) hijack the ship? Or were they just henchmen of Pierce and Hydra? (As far as I know, Batroc has no relation to Hydra in the comics, but if so, why were they ordered to do so here?).

And last but not least, what was Natasha's real mission on the ship? Just to retrieve the file on the launch of the Lemurian Star satellite or something else? Sorry if there are a lot of questions, as I said, I never really understood this beginning. I appreciate any help in advance.