I had asked the same question as well and in fact I bought a 3mm, 2 days before my sipadan trip. However, after the 1st dive with wetsuit, I realised how uncomfortable it was because the water is so warm. I took out my wetusit and dive with rashies. But then again some of the guys on the boat has no problem diving with their wetsuit. I think if you are one of those that dive with rashies in Tioman and Dayang just like me, then you dont need wetsuit in Sipadan. And yes there's thermocline there but it is actually nice, just like having a cold bath after sauna..but then again never hurt to have a wetsuit handy.