I don't think there is a specific point Ken. It dilutes over a section but I have no idea how long it is. I have seen live crabs at Putney, saw a Heron catch one and was amazed when it swallowed it whole. Saw a large group of baby flatfish, definitely sea fish. And a Red Gurnard in a puddle at low tide. Recently saw a half dozen Salmon at the mouth of the River Wandle at Wandsworth. But they go into fresh water to spawn. A seal at Richmond upon Thames does not count because it can live in fresh water. Many years ago I watched some dolphins at Battersea. The River Thames is an entertaining place.