I see no way that salt water can affect the integrity of diamond surface. Mechanical abrasion seems unlikely, given that diamond is among the hardest substances known; and diamond is incredibly resistant to corrosion.
That said, salt water can leave a residue on the diamond that affects its reflectivity, shine, etc. Such residue could be pretty difficult to clean off. Salt may also corrode the metal surfaces of the jewelry, including the small parts that hold the stone in place. Also, if the diamond has any coating to enhance it's optical properties, they may be prone to mechanical wear or corrosion. So, as a general rule it's probably a good idea to leave the diamond ring at home when you go swimming in the ocean.
But as the for the diamond itself, I think it's safe from salt water. If I took a diamond ring into the ocean and noticed the surface was worn afterwards, I think you might have to go have a conversation with whoever gave it to you and told you it was a real diamond