As noted upthread, what you get for a "hydro" varies, not just hydro shop vs dive shop but every shop does things a little differently.
The very best dive shops do it this way:
1) They visually inspect your cylinder right when you bring it in, and if it's a basket case they give it back to you right then and don't charge you. (Hydro places invariably charge for testing service whether or not the cylinder passes)
2) They put an identifying sticker on the valve and cylinder so you can be sure you get the same one back. (Hydro places, while making some effort to keep things straight, consider valves and cylinders to be fully fungible)
3) They take the cylinder to the hydro shop, and go back and pick it up (so you don't have to drive there twice. There are far more dive shops than hydro shops)
4) They visually inspect the cylinder themselves, and put a VIP sticker on it.
5) They replace the neck O-ring unless the hydro shop already did it.
6) They replace the burst disc.
7) If the cylinder has a yoke valve, they replace the yoke O-ring.
8) They fill the cylinder with air.
If you have a twinset then a good dive shop will break down and re-assemble the twinset, though most charge extra for this.
There is a case to be made for disassembling and cleaning the valve at hydro, but I'm not aware of any shops that do this as part of the hydro -- it must be separately requested and there is an additional charge.
