It's just another aspect of flying that gets compensated for. Dropping at a constant height is important, so a natural reduction on the collective keeps the machine steady. Filling a bucket is the same as lifting a load, with constant collective pull. Tank fill is slower than a bucket fill, and most of the time the tank fill will be stopped when the hover torque gets to about max less 5-10%, giving reserve to translate away from the water source.
S64 fills in about 30 seconds with the donkey D., about 18 seconds with the boom arm. Since the boom arm is dragged across the water surface, it's similar to lifting a load, just keep pulling power.