Always worth carrying a spare spike I reckon. If you are on a canal and your budget runs to it then also get couple of "staples" (aka armco hooks) and a couple of mooring chains. They can be used anywhere there is metal piling and are quicker, easier and more secure than spikes.
I always think mooring chains are better on Armco than the hooks; before I saw the ones with rings on each end, I used a length of chain with a karabiner to connect the ends and slip the mooring rope through - never had any problems with that!