There is currently no standard tool for visualizing POP output. There are two freely-available software packages that can be used.
Ferret is a visualization tool developed by Steve Hankin at NOAA's Pacific Marine Environmental Laboratory (PMEL) in Seattle. It is designed specifically for visualizing ocean model output and data. Ferret is constantly being improved and extended, and there is a very active email-based user group. For more information about Ferret, check the Ferret website
http://ferret.wrc.noaa.gov/Ferret/.
Other free visualization and analysis packages are available from the DOE-sponsored Program for Climate Model Diagnosis and Intercomparison (PCMDI) at Lawrence Livermore National Laboratory (LLNL). Although originally designed for visualizing atmospheric model output from the Atmospheric Model Intercomparison Project (AMIP), many are applicable to ocean model output and future ocean analysis tools are being added. For more information, check the website
http://www-pcmdi.llnl.gov/software/.