some good advice.
I haven't paddled sea kayaks for several years now but when Idid I got numb bum and numb legs. problem was alleviated by placing a rolled up foam sleeping mat under my thighs just forward ofthe seat (happy bottom fromCLC). as well,I developed tendonosis from poor paddling technique. i have moved on fromsea kayaks andhavebeen paddling HP surfskis. these beasties demand good forward stroke technique, hip rotation & leg drive. They also have a hump inthe middle of thecoackpit that amongother things, provide good support to legs.
summary,try thepadding under the thighs as suggested and review your paddling technique http://www.youtube.com/watch?v=lef_ut8n0ms&feature=youtube_gdata_player
Cheers