The River Ness is only about 10km in length, so it is not very far from the north end of the loch to the sea.
However, there are several sections where the river is quite shallow and I am sure that a Killer Whale would have great difficulty in navigating from the sea to the loch.
There is of course an alternative route used by various medium sized boats, the Caledonian Canal, which runs parallel to the river for most of the way from the loch to the sea.
The problem with that is, I don't think that a whale would go un-noticed passing through the locks!
JimB