Obvious don't go when mao mao lah. Then we'll be reading about you in ST: "Singaporean man falls into Bangkok canal"
Ride is scary first time but I've never heard about accidents before. Some boat captains are like the old SBS non-aircon bus days - macham F1 driver.
Where to take it? An easy one is the Asoke pier which is next to the MRT station - exit MRT, walk towards Sukhumvit direction and you'll see a path that leads to the canal.
Just jump on the boat (make sure it's going in the right direction). First time, don't enter via the bench seats area. Enter the middle, where you have a bigger "landing" area. Then wait for the conductor to come and you pay. Stand in the middle to avoid the water sprays. For added bonus, go at peak times to get body-to-body with sao sao OLs.
Taxi closes down at 8pm I think
You can take it to the Pratunam pier which I think is 2 piers away, after the Nana pier. Then you can say, "I've done it, never again" and go back to your luxury pickup