Through the magic of Googling "Persian Gulf salinity" it seems like it's more that it's a shallow Gulf in a dry area so it has significant evaporation. Desalination does effect it but it's only a few percent of the total evaporation (which is still surprisingly big) and doesn't sound like the main driving factor or an imminent ecological concern.

https://www.frontiersin.org/journals/marine-science/articles...

https://www.sciencedirect.com/science/article/abs/pii/S14635...