It's easily possible that he was dehydrated in his fight with Pacquiao, but that was not the only reason he fought the way he did. Freddie Roach is very good about knowing when boxers are reaching their limit and knowing how to fight under favorable conditions. Historically, fighters who have gone up in weight often have trouble when they try to move down in weight again. Pacquiao vs De La Hoya is a prime example of a fight that Freddie Roach picked under favorable conditions. De La Hoya had moved up to middle weight, but had to drop back down to welter to fight Pacquiao, which was no easy feat as he was already pushing on into his later years of his boxing career. I believe that even if De La Hoya was not dehydrated, Pacquiao would still have won, but it would have been a much more competitive bout. Likewise, I believe that Cotto was dehydrated as he has a history of struggling to make weight, but even if had not been dried out, I believe Pacquiao would still have won, but it would definitely have been a more competitive fight, with the whole fight perhaps resembling the first 3 rounds of the fight.