Yes, there is an online option to book however I found it easier to seek assistance at the reception of our accommodation.

Please save yourself the trouble and the 550 Baht (or however much) for a night of brain-numbing boredom. There is absolutely nothing to see along the river at night and no narration on the cruise. To add insult to injury, the food they serve is inedible and smells of fertilizer. Well, if you enjoy the romantic sight of an expressway and a bunch of 7-11s, you might perhaps enjoy the cruise.