Got this off a Google search:
HYDRAULIC FLUIDSFile Format: PDF/Adobe Acrobat - View as HTML
respect to submarine operations is the preferred use of hydraulic fluids miscible ... Another hydraulic fluid, also containing ethylene glycol (94 - 95 %) ...
www.chemistry.usna.edu/navapps/PDF/Hydraulic.pdf - Similar pages
Link talks about the different hydraulic fluids used in submarines - your fluid could be glycol based (50 - 95%, rest water and amine corrosion inhibitors) instead of gylcerine.