What you can do with the Arduino that is very useful is to receive signals from these probes/circuits and make them available for display, data archiving and control.
There are a number of folks doing things with these probes to control their aquariums. Just search on Aquariums. I've seen handmade ph, temperature and conductivity probes. I recall one user that has even custom built PH, ORP and Conductivity Shields. (search Arduarium)
BTW, Salinity can be calculated from a conductivity probe. In fact, Conductivity, Salinity and Total Dessolved Solids can be derived together.
Atlas Scientific has nice stuff (I am using it for two projects that involve Arduino for prototyping). It is expensive, but is pretty much system agnostic (they standardized on serial communications to interface with their sensor/circuitboard combos.)