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: