Unless you are an electrical engineer, I do not see conductivity as a DIY.
If you are, there are many probes available from places like Cole Parmer, and they do show the appropriate range in their catalog. For seawater, you'd want up to at least 60 mS/cm (60,000 uS/cm). Some probes have 2 and many of the best ones have 4 electrodes. They all typically also incorporate a temperature sensor since the value must be temperature compensated.
I discuss the method and complexities a bit here:
Using Conductivity to Measure Salinity
http://web.archive.org/web/200406040....aspx?aid=1804
and this one has more on conductivity:
What is TDS?
http://www.reefkeeping.com/issues/20...ture/index.php