It shouldn't be too difficult to understand...elephant seals prey mainly on squid and fish, both are poikilothermic which means their metabolic rate varies dependent on the ambient environmental temperature. So if surface waters get warmer, as the heat convects downward in the ocean, the prey have to dive deeper to maintain their optimal metabolic rate. Therefore the predators will also have to dive deeper.
Derp.
That's what the science would predict. Now they will test it. There are many successful predictions that occur just like this. Much of climate science was predicted by theory just like this before the tech was in place to make the observations necessary to confirm the hypotheses.