Is to simply slow down.
Yes, hydration is important.
As is protecting your skin and fueling properly.
However, if you go out too fast during your race in the heat, no amount of hydration will save you. In fact, it's more likely to kill you.
By pacing accordingly and slowing down for the first section of the course, you will be in a fine position to finish strong (even if it's slower than you had hoped).
A good example of this is Team SKORA member Taylor's 2012 Boston Marathon. During this event the high temperature was 89 degrees and over 100 runners visited the hospital. However Taylor did a wonderful job of restraining himself early on and had a very good run. You can read his race report here!
So, how do you pace and hydrate in the heat?
Jeff Galloway has a great tool for helping determine pace in the heat. In Galloway's Book on Running, he offers the below chart for adjusting the pace that you race at, for the heat. He does mention it is completely anecdotal, but has used it with likely thousands of runners.