just got back from Sipadan a few days ago. was there for a week and had sunny days and calm seas to start with but constant rain, wind, and rough seas the last three days. viz was between 15-20m at some deeper sites to a hazy 5m in the shallows on some days. most articles i read mentioned April to September as dry season so presume the weather was due to a freak storm somewhere. that said the diving was still great. lots of reef sharks and turtles, and saw the schools of barracuda, trevally and bumphead parrotfish Sipadan is famous for so overall no complaints.
was there last year in August and weather was patchy then as well. but overall better conditions with 30m viz on some days.
btw, if you are on a tight budget (like me) then the Seaventures platform is great value for money. rooms are very small and basic but the staff are helpful, food is great, and the diving under the rig is easy and very good. its a great place for a night dive! but wont recommend Seaventures if you are going with a non diving partner or want a lot of creature comforts!