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The average conductivity of sea water at 20degC and a salinity of 35g/kg is:
4.788 S/m (Siemens/meter)
http://www.kayelaby.npl.co.uk/general_ph…

The resistance is the reciprocal of the conductance, so:
R = 1/(4.788 S/m) = 0.2089 ohm/m <===ANSWER
at 20 degC and a salinity of 35g salt per kg water.

If you place two probes 1 meter apart in seawater under the stated conditions, the resistance will be 0.2089 ohm, according to the calculation and published conductance data.Asker's Rating:Asker's Commenterfect answer!
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looks like the readings move to the milliOhms once the water has some salt introduced, so you would have to work with op-amps to detect small changes in a certain range. Also mabye a reference to calculate from...such as .35 Ohms(some a/d reading) equals 200 PPM,parts/million.

that link seems broken, just a search for conductivity of salt water.