All the replies above are on the money. Even if you see them at Gordo Banks, which I did at around 125 ft this past November (15 to 20, at a distance that was not shootable, and for all of maybe 10 seconds), it is probably not the iconic hammerhead wallpaper image that you may envision. In other words, it is not likely to be in the numbers that will fill your field of view. But then again, it is the sea, and anything is possible.
Socorros, as suggested by Sharkie Guy, is a better bet. I hear that the dive site called El Cañón in the island of San Benedicto has been very good this year for HHs and other stuff. And I just received an email from Pelagic Life - owner of the Solmar V and Vortex liveaboards - announcing that they are discounting all their April thru June trips by 50%.
If you are locked to being on land and only those dates, then the only other place to try for HHs is out of La Paz at a dive site called El Bajo. I didn't see any there this past November tho - some people saw 1 or 2 - and the same caveat as with Gordo Banks applies: not likely to be in large numbers, if at all, nor close enough or long enough to shoot.
