Probably not the best.

This is from my travel agent's website:  "For adventurous divers,
Cocos is a year round destination. However, there are two distinct
seasons, each with its own merits. The dry season runs from November
to May. It will provide calmer seas, and a more comfortable crossing.
It's also the time for silky sharks, and large schools of Mobula Rays.
Water temperature will range from 78-82 degrees Fahrenheit.
Hammerheads will be present, but in lesser numbers than during the
Rainy Season, which runs from June to November. The rainy Season is
the time for the large schools of hammerheads, but also brings rougher
sea conditions and a tougher crossing on the 30+ hour run from
Puntarenas to the first dive sites. During the Rainy Season, water
temperature will range from 76-80 degrees Fahrenheit."

http://www.islandream.com/coco_rpt.htm

I went in July, but wasn't overwhelmed by the numbers of hammerheads,
and they didn't come in close like in the Galapagos.  Besides the
season, it's how the currents and tides are running (moon phase), el
nino/la nina events, etc.  Always a crap shoot.  Howard Hall endured
over 50 hammerhead-free days while filming his IMAX movie there.