By how large a boat? My understanding is that cargo vessels can berth just below Nantes, but that is as far as they go.
I've motored alongside the major part of the lower Loire, but it has never struck me as good for boating - shallow, and fast flowing. I have only ever seen one power driven odd looking contraption on the river up-stream of Nantes, and it was making very slow progress against the current. The Vienne might be better, but that's not much good if cannot be reached.
You will get up in a canoe, provided you're prepared to do a bit of carrying. /forums/images/graemlins/wink.gif
There's some rapids and IIRC a weir near Le Fresne Sur Loire which is about 30M upstream of Nantes, but realistically I doubt you'd even get that far in anything drawing more than a rib - take plenty of shear-pins..
I have sailed (not motored) up the Loire as far as Nantes from St Nazaire, anchoring once halfway for the tide. After Nantes the mast has to come down.
The tides are impressive but there was alongside berthing at Nantes, which is a good base from which to explore all those Muscadets hic..Probably got full service marinas now..
It looks navigable by flat-bottomed punt with low air-draught. I've only ever seen very shallow-draught boats thereabouts. Not sure if there are any weirs between Saumur and Nantes.