Is the PCT of the nephrons of the kidneys of marine vertebrates to minimize the natural tendency for exosmosis?
First off, I agree with @WYSIWYG - marine life has adapted to conserve water, not electrolytes. Osmosis in sea water draws water from the body and hence their problem is to keep the water in, not the salts. 

Osmoregulation in marine mammals is accomplished mainly via differences in hormonal regulation of the kidneys. The histology and morphology of the kidneys are different, but the adaptations have more to do with adaptations to diving/pressure than anything else. Instead, most marine mammals take care not to drink too much sea water and obtain their water from metabolism and foods (Ortiz, 2001).  

Reference
- Ortiz, J Exp Biol (2001); 204: 1831-44