>>955301Any sit-on-top will do for ocean use - mine is 4.11 meters long and I constantly see 3 meter kayaks out on the bay. How big are you? If you're small I'd say to go for a 3 meter 'yak, if you're average or big I'd recommend 3.5-4 meters. If you go smaller or bigger that's OK. The major important thing is that it's sit-on-top.
>any dangers with second hand sit on top kayaks?Absolutely not! Buying used is a great way to start - who wants to drop a grand or more on a sport they might not even like? Just make sure you can see detailed pics or the actual 'yak before you hand over your cash. Make sure there are no holes other than the bilge holes, but otherwise I highly recommend buying used for any first-timer.
Oh, one more thing - and this is a must - you absolutely have to post back here with pics of your first time out!