Just finished reading Steve Van Beek's "Slithering South", about his journey on a little sampan, from Northern Thailand to the mouth of the Chao Phraya, back in 1988.
I plotted his route on Google Maps: https://www.google.com/maps/d/edit?mid=1CUh9tW7klW587sDoG4LmKyXXUHuJ3gI
Great book.
From kayaking here and there I feel like not much has changed since his journey.
That said the Chao Phraya river is much cleaner nowadays, and Khlong U-Thong in Ayutthaya is fine.
:)