Depends on the canal, I cycle along the Union Canal - it's 3'6" but the Forth & Clyde Canal which it connects to is nearly double that at 6' (and a lot deeper in the locks). The Crinan Canal is 8'10" deep and the Caledonian Canal 12'6".
I've sailed through them all (apart from the Union Canal as my boat draws 3" less than the maximum, not good of there is a shopping trolley in there...) so know them fairly well. Typically it's the sea linked canals that are deeper in order to accommodate the larger sea going vessels.
Some of the craft that go through the Caledonian and Crinan canals are quite big, I got stuck behind the attached ship - 'Lord of the Glens' when transiting the Caledonain Canal a few years ago.