Both low water (mid-Nov to end-Aug) and high water (end-Aug to mid-Nov) have similar onshore excursions but the sequence may differ.
The main distinction is that the water levels only influence the itinerary in Cambodia (between Siem Reap and Phnom Penh), while the Vietnam itinerary is unchanged.
In particular, Mekong River cruises can make a lake crossing in the high-water season and visit many attractions in Tonle Sap. Ships can also dock directly at the Siem Reap pier. During low water season, the dock will be located at Kampong Cham.
Note: The exact dates of the high and low seasons are determined by rainfall and water levels each year. Mekong cruise lines will adjust their schedules on short notice.