One of the Passover rituals is to dip parsley in salt water. I’m not sure why.
At every Seder I have ever been to the salt water was made at the home. It is easy to do but sometimes it is done wrong. Usually not enough salt. My suggestion is to sell premixed salt water in a beautiful bottle with a connection to Israel. It could be purified Red Sea or Dead Sea water.
A test market could be done by having a youth group like United Synagogue Youth try to sell it in one area. The unique selling proposition is: Convenience, Purity, Connection to Israel and Tradition.