In my experience (and that of my friends) the Nautilus chart wasn't so accurate.
Things to keep in mind for the time of year to go are the animals you will see, to an extent, and the water temperatures. Conditions are a bit of a toss-up but tend to be more affected by moon cycles than anything else.
November and December are the months with the warmest waters. I went last year mid-to-end of December and was comfortable in my 5 mm with hoodie (I wear a 3 mm with hoodie in 30 degree waters so I run very cold) - temps around 25-26 degrees with thermoclines down to 22 degrees in parts. Weather was calm and we really lucked out with both crossings being extremely calm. We were there over full moon which made the surge/swell really gnarly at times. I found the currents to be totally manageable and even non-existent some dives (very nice for those dives where you just chill in the blue and play with mantas for 45 minutes). Bad surges did rock us about smacking people into rocks and other things and I found it hurt my ears since we were bouncing up and down about 8 metres. They tell you not to fight it but sometimes you have to and it can make for frustrating and less than pleasant dives. If I were to go back, I think I'd avoid this moon stage. December was also supposed to be whale shark season but we saw none. We did a lot of hunting for hammerhead schools but came up empty (we did see some individual scouts here and there). Could be just luck or the warmer water temperatures. Tons of bigger sharks though (galapagos, dusky - tons of white-tips) and endless supply of oceanic mantas plus big schools of fish (jacks, tunas etc). My girlfriend has been twice in March where the water is much much colder but saw whale sharks and schools of hammerheads plus it is humpback season (this tends to start end of January). In-water encounters are very rare and only allowed if you happen to bump into them by accident (you can't intentionally get in the water with them) but surface encounters are common and you of course will hear them singing underwater.
