There are many issues with using either salt (sea water) or produced water for frac fluids. Sea water has a typical level of about 3.5% total dissolved solids TDS (by weight) and produced water 10% or more. Carbonate and sulphate anions can reach their limit of solubility quite easily . Both sea water and produced water are in a chemical equilibrium with these anions. The main cations that in sea water are sodium, potassium, calcium and magnesium. Calcium and Magnesium will be at or very close to their solubility limits ( hence limestone rock and sulphate deposits on coastlines). pH, pressure and temperature changes can also cause precipitation.

High TDS water causes issues with the preparation of frac fluids which vary in composition by include cross linked gels, guar gums and polyelectrolytes which are all impaired by high TDS prep waters. Even more important is the concentration for cations in the formation water which will be at the limit of solubility for Calcium, Magnesium, Barium and Strontium.  Barium and Strontium both have very low solubility with carbonates and sulphates. Any precipitation of salts in the formation will impede the flow rate. pH control is also critical and frac fluids are typically alkaline to help reduce corrosion. 

For this reason fresh water, which is typically less than 0.08 % TDS is preferred.

I have attached a link to a paper from the ASC (American Chemistry Society) which goes into detail on this subject. It is very technical but if there are any questions I will do  my best to explain.

https://pubs.acs.org/doi/pdf/10.1021/acsomega.3c05145?ref=article_openPDF

For about 6 years I was heavily involved with water treatment for 0il production, refineries and petchem plants before moving  into refining and petrochemical plants technology.