The megabags are pulled by ocean-going tugs designed or sized for the appropriate voyage length, eg, 10-days.
Depending on the loaded capacity of the bag, there is an optimum tug boat size. This optimization is important and velocity is the major variable. If speeds are low, operating costs are similarly low but more capital equipment is needed to maintain a desired delivery volume of fresh water. So optimization is a play between the cost of capital, including equipment life, and operating costs, primarily fuel.