On the west side of Peconic ave after the small falls is where it turns brackish. On the east side it is brackish but turns to salt very quickly due to the vast expanse of water it flows into. I think out by the marina in Jamesport on the north side of Peconic Ave is brackish. Never fished there but I heard of White perch up in there. It gets pretty low on low tide but I am sure if you were adventurous and willing to get muddy you may find some holes back there. Best bet would probably be a canoe or small boat at high tide. It doesn't look all that accessible.