Although this is quite an old post, I still wanted to share my experiences as I saw it before I went and got quite afraid I was about to be having a horrible trip.
To put it in a nutshell: It was awesome! I went to Cocos in October, during what many experts believe is going to be one of the strongest El Ninos ever. The water was indeed very warm (28 degrees Celsius on most dives with the occasional thermocline between 20 and 40m and around 22 degrees below that). Nevertheless, we had loads of hammerheads, also big schools (the biggest with at least 300) and a total of 7 different species of shark (Hammers, Tigers, Galapagos, Silkies, Silvertips, Whitetips, Blacktips). Also, the hammers weren't that deep with some of the schools ranging almost to the surface.
All in all I guess it's just a matter of luck but from what I've heard the trips before mine were pretty great as well.
Have a look at my video for some proof: https://www.youtube.com/watch?v=y0KyjuLri9k
Cheers