Are you travelling on your own? I stayed on the rig for either 4 or 5 nights, and managed to get to Sipadan every day bar one, BOOM! that was at the end of August. Plus, the permit is for the whole day, not just 3 dives, so the centre should be able to get you 5 dives on Sipadan in one day, though they will charge you a lot more for the boat fuel to make a second trip. Second trip off the day is more expensive, but often less popular and therefore more exclusive, therefore you don't have to piss ass around waiting for the inexperienced divers to descend/reascend in current while you wait. My favourite Sipadan dive was one of these afternoon dives, where after finishing our dive we near bled our cylinders... (ok to about 20bar), in 3-5m of water inside a huge school of jacks, only 4 divers in the water.
Yes that's probably enough days, I think I totalled 9 days, but could have easily stayed longer. Play it by ear. There is plenty of cheaper accommodation on Mabul if you decide to stay longer.
With that many days you could complete your intro to cavern cert, and dive some of turtle tomb (not the tomb bit, but the entrance). That was my intention, and why I booked 9 days. But I couldn't find anyone that I was comfortable training with....
I can't comment on whether more days would mean more Sipadan or not. It could potentially mean less Sipadan as they know that you are there for longer and might prioritise trips for people who are there for fewer days.... Permits are a massive pain in the bum.
Nic
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Also visibility and suitability of specific dive sites at Sipadan seemed to change by the day/ hour when I was there, so I would say that the longer you are there, the more chance you would have of visiting everything that you want to and getting the most out of the sites.
I didn't have great visibility when I visited. One one dive I saw a leopard shark. I would have paid for 3 more days at Sipadan to see another one, it was beautiful and I had never seen one before, but *cough* I already had a few mins of deco, no buddy, etc lalala, and therefore had to watch it swim into the current below me and pray I saw another one *cough*
Guide to diver ratio is not always good, neither is the balance of experience within the group...
If you have a buddy the guides generally mumble something during the briefing about 'if you surface separately from the group' which the more experienced divers took as a green light to signal to the guide, 'we're going off on our own'
Nic