Inexpensive boats that travel south of Sathorn Pier?

I’ve ridden the Chao Phrya river boat from Sathorn to Nonthaburi and would note like to explore the river toward Samut Prakan / the delta. Are there any inexpensive boat going that way? The only one I found was to Asiatique—not very far down. I’d read that the Chao Phrya Express route was to be expanded south, but I don’t think that ever happened.

I’ve ventured down to Samut Prakan via BTS, going to Pak Nam, then to Kheha to visit The Ancient City, but would really like to see what else is on the river from its mouth to Sathorn.