A helmet is an essential kayak safety item whenever you are kayaking in locations that put your skull at risk: surf, tidal rapids, whitewater kayaking, kayaking in sea caves, rock gardens and gauntlets or near reefs and bombies. Without one you are risking those brain cells you can't afford to lose, or worse!
It might be hard rock that impacts your head but even in deeper water, where there are waves there is the chance of coming out of your kayak and coming up on the wrong (down-wave) side - it could be your own kayak that hits you!
We like the Predator Helmet - the current model is the Shiznit. You can get bright colours for greater visibility or darker shades. The Shiznit gives good coverage to the sides of the head and it has a decent brim - this not only protects your face better than a brimless helmet but takes away the need to wear your sunhat/cap underneath.
With a good helmet you can tackle your adventures more confidently - like learning to surf tidal rapids in Surge Narrows, near Vancouver Island, Canada on our expedition in July 2012: