I've done the Cache from Ferguson to Kawnipi in August during average/ normal water levels with no problems at all. From what I saw the water levels would have to be exceptionally low to prevent travel. Some of the portages may be a little longer but no big deal.
The portage from Ferg. to Cache will be easier during a dry period. It is low flat and very mossy. Probably slow going when saturated with water. It was dry when we did it and easy but long.