Given an equal amount of sun exposure... Will someone in a wet
saltwater environment vs. freshwater receive a greater/lessor sunburn?
Example: A sailor exposed to wet conditions (sea spray) on an ocean
vs. a lake.

My guess is the direct effects are the same plus or minus a few parts
per million. There may be a dozen indirect effects such as salt
crystals are irritateing to the skin especilly skin injured by sun
burn. The salt crystals will reflect some of the sunlight reducing
your exposure, but your question specifies equal exposure. Trace
ammounts of a wide variety of things are found in either salt or fresh
water, some of which may add to the skin damage the sun burn caused.  
Neil