Yes you are right mamakpenang! Viz depends on tide and tide is all about the position of the moon and the sun. Hijrah calender is based on the moon that is.
We were there during the end of neap tide, not much of water movement thus vis is better, but I'd say fish is less during this time as not much of movement of water, less oxygen.
mmi: Jarak is about 60km off Hutan Melintang jetty. It's a 5 hour boat journey traveling at about 7 knots. On Kaleebso, you don't really feel it. Take a nap, watch a movie, play some card games... and you are there.
As for 15m viz, it's just an estimate. But I can see the bottom of the sea from the surface and the depth is 12m. It's good for Jarak.