Most have ATC.
Most of the refractometers sold in the hobby are called salt/brine refractometers. These are perfectly fine for the hobbiest. I would recommend buying calibration fluid to calibrate it rather than using distilled water. This will decrease the margin of error greatly.
If you want to shell out some extra cash, then you can buy a refractometer made specifically for seawater.
I would go with the one on this site. It is a good price and works just as good as any other salt/brine refractometer.