Yeah it does seem like having a dedicated, bridge "Science Officer" is kind of "Captain's Choice". Some ships have them. Some don't. Data does just kind of act as a science officer, but it's more so just because he's really good at science.
I also think that the E-D's science crew is huge, and i'm not sure there is something like a "Chief Science Officer" onboard. Rather, I think they just treat each department as it's own thing. You don't really need a "Head of EVERY SCIENCE!", you have the Chief Medical Officer, Head of Stellar Cartography, Head of Anthropology, etc. From there I think they the individual heads would report to Riker, but given the often informal nature of Starfleet, Data might just handle most of the science-based things.