Optional equipment that may be quite useful are repair kits, river knife, rescue gear, tube tent for shelter, or a space blanket bivy sack. I would weigh the benefits against the extra weight though. Heavy boats can be more challenging to paddle and certainly are a pain carrying. A cell phone in a Pelican box is a really bright idea though as it doesn't weigh much and may help save someones life.
Outfitting your boat and helmet are generally essential in this sport. A loose fitting helmet often exposes the forehead to impact and concussions. A loose boat is much more difficult to control. Here are some useful articles on outfitting:
