I haven't paddled a sea kayak for a number of years now but when I did I would throw a spare paddle on the back when going off shore. Nowadays I paddle a surfski and nearly always solo and have often thought that a pair of rolling sticks in my aft deck bungees might be worthwhile.
Eg. http://blackdogkayaks.com/forum/YaBB.pl?num=1202921219

I remember seeing a pair with a nasty looking pointy bit on the end which I thought might come in handy if a shark got too friendly