It's not the right kind of refractometer, the scale is way to big. Seawater averages 35 parts per thousand which would be 3.5%. That is equivalent to 1.0265 SG.
I'm not sure if that will measure the refractive index of seawater correctly though. It should be close enough if you keep it in between 3% and 4% but keep in mind that 3% is about 1.023 and 4% is about 1.030 so you may be better off with something that is more precise.
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