Mark,
 
The easiest (but not the cheapest) way to measure salinity is by using a
refractometer, that is by measuring the refractive index of sea water, which
changes with salinity. There are refractometers on the market that are
calibrated to measure salinity directly. (Otherwise, you measure refractive
index, then you have to look up the corresponding salinity from a table).
They are very easy to use: you simply put a drop or two of sample on the
instrument, hold it against a light source, read the salinity. If the
instrument doesn't automatically compensate for temperature, you have to do
it yourself, or take all your measurements at the same temperature. They are
small, light weight & difficult to damage with reasonable care. They cost
about $200-300. I bought one about 2 years ago from a company in Florida for
about $200. That was the cheapest I could find in the market at that time. I
found the company on the Internet, but now I can't think of their name
(Aquatic Eco Sciences, or something like that; an aquaculture company).
 
An hydrometer may also be used to measure salinity. They are much cheaper, at
the same time more difficult to use & less accurate. You also need a lot more
sample, because you need to float the hydrometer -a long thermometer-like
instrument- in the sample.
 
Another accurate & practical way to measure salinity is by measuring the
conductivity of sea water using a conductivity meter. They tend to be more
expensive & require more maintenance than refractometers.
 
A.