Welcome to the Humminbird Side imaging Forums Helixbob.
I don’t know why the Si sonar would not work in brackish water. If there were a lot of air bubbles mixed in where the rivers run into the bay I could see it causing problems but not due to the salt content. I’ve fished in the Mississippi river where it flows into the marshlands down by Venice, LA. and the Si units had no problem there. I’m sure that there are many guys on here that have used their Si units in brackish water and would tell you the same, if they see this post.