Welcome to cheapest transport in Thailand for less than a dollar ( 60 Cents) , this is call Ramkhamhaeng ferry or boat.
This line comes from Wat Saket and goes all the way to Bang kapi area. There is an interchange with Mrt blue line and also Airport link at “ Ramkhamhaeng “ station.
In this video you will see all the way and stations and also Nice Path for cycling and jogging along the river.