Every major pier has small cross-river ferries that just shuttle back and forth for ฿5. Tourists always take the expensive express boats or walk miles to find a bridge.

At Saphan Taksin pier, there's a tiny ferry that crosses to the Thonburi side every 10 minutes. Same at Tha Chang near the Grand Palace — Little blue boat crosses to Wat Arun in 3 minutes for ฿5 instead of the ฿200 longtail tourist boats. Just follow the locals carrying groceries, they know where the cheap boats are.