Hi Henry,
I was in Mabul/Sipadan in Feb. this year. If you want to dive in Sipadan, who doesn’t, you should stay either in Mabul or Kapalai or the oil rig. Smart of Mabul might be the best bet. It takes about 30-45 minutes from Mabul to Sipadan by speed boat, depending on the weather. Do not foeget to bring a wind-breaker as the ride could be rough if it rains. There are only 120 diving permits issued everyday for diving in Sipadan and usually the diving center let you have two dives there in the morning. You should try to leave as early as 5:30 as it is the feeding time and you will be able to see more. These islands are certainly not recommonded for foods but I survived in any case.
Safe and fun diving!
BJ