1. What do you want to count: total hemocytes (eith DAPI or Hoescht), viability controls?, some marker that a flow cytometer can detect (i.e. a protein marked with FITC)?
2. Independent of the 1st question i think that PBS 1X pH = 7,4 is good for flow cytometry. Filtered seawater could be an option but i think that the high concentration of ions (Mg2+) could be a problem.