the monster manual describes sahuagin as worshipers of sekolah, great demonic shark, who is an exarch of melora. however, given their chaotic evil alignment, i instead would have them worshipping the demon lord, dagon.
i supposed that i am inclined to see most, if not all, chaotic evil races worshipping demons rather than gods because, in my view, none of the gods are chaotic evil, with one explainable exception--tharizdun. originally, tharizdun was not chaotic evil. he became chaotic evil when he was overwhelmed by the shard of evil. anyway, he is locked up and largely out of the picture now, despite what his looney worshippers might believe. gruumsh, in my view, is a demon lord, not a god. so that leaves no chaotic evil gods.
so, who among the demon lords would best serve as the patriarch of sahuagin? well, an aquatic oriented demon would be fitting. dagon is an aquatic demon lord. also, from the monster manual 2, we know that dagon's temples are underwater. what else do we know?
i) dagon's [land-based] cultists reside in isolated seaside towns and villages;
ii) sahuagin dwell along coastal waters;
iii) villages prone to [dagon's] cult descend into savagery;
iv) sahuagin slaughter and devour anything they can catch, raiding coastal settlements in the dead of night;
v) a sahuagin kingdom can stretch along an entire seacoast, and the sahuagin sometimes trade for goods, but they are more inclined to take what they want by pillaging coastal communities and merchant ships; and
vi) dagon's temples include air-filled chambers and caverns for his terrestrial followers.
although not a completely compelling argument, i do think that the above information makes for a good fit between sahuagin and the demon lord, dagon.
i just remembered a great little adventure in dungeon 156 called the last breaths of ashenport. i think that the fish men of dagon easily could be re-skinned as sahuagin.