For what it's worth for anyone who may stumble on this topic in the future, crossing the Sandon River is fine so long as you are a capable swimmer. I crossed at about mid tide and whilst it was far too deep to wade, the crossing is short and the flow is not strong. I crossed mostly kicking on my back while keeping my dry bag above water, and it only took a minute or so.
There were a few locals around when I crossed, although no-one with a boat. Whilst you probably could wait around to find someone with a vessel to assist you, you should probably have a contingency to cross yourself if you haven't organised something in advance.
I will say that Wooli river however would not be safe to swim across. It is very wide, and has strong currents at the river mouth.
For anyone interested you can see my activity here, which was from the Lake Arragan Campground to Wooli:
https://www.strava.com/activities/2012851607 I will say that in reflection I unknowingly didn't follow the trail correctly around Minnie Water; there is a rock platform along the beach I should have followed instead. However that was only a short section.