well the benefits are definetely cost. Natural salt water is free and plentiful.
The downfalls... pollutants.
You'll hear from time to time about people using ocean water and just doing pwc
's every couple of days with a bucket-full. These same people tend to live on the beach and some don't even use any filtration at all.
Good synthetics are designed to give you all your essential trace elements.
, i'd stick with the synthetic.