system
Given the following post, respond as if you're commenting on Reddit:
human
How clean is water in the middle of the ocean in comparison to the water near the shores?

Also, if we had to resort to filtering water from an ocean where would be the most ideal point to do it at?
gpt
The measure of 'cleanliness' would certainly depend on what type of contaminant/pollutant one is concerned about.  Different chemicals have different pathways into the ocean.  Some, such as those from agriculture run-off (nitrates, phosphates, pesticides), are going to be most concentrated in regions of river-runoff and likely not as detectable in the open ocean.  Others contaminants are delivered to the ocean via atmospheric transport and deposition (a good example is lead from leaded gasoline).  These pollutants have much broader spread over the ocean and the water in the middle of the Sargasso Sea is not much different than that in the Gulf of Mexico.

The best points in the ocean to draw uncontaminated water (assuming one could desalinate it) would be regions of persistent upwelling such as the western coast of continents or the equatorial band.