It's not like DF users automatically die when falling in water. Akainu never lost consciousness nor did it ever show him actually falling into the water. From what I gather Akainu blasted himself into the surrounding ground before hitting the ocean. From there he melted his own tunnel to reappear in front of Jimbei.
Besides even if he did hit the ocean, there were plenty of other marines who also fell that could have fished him up. It's been shown that even when wet, Luffy still retains his powers. He punched through the bubble coating to grab Sanji and had his head stretched above the water when being saved by Ganzo and Nojiko. Akainu's one of the strongest characters in the series, so I bet even after being wet he's still have plenty of fight left in him.
And Akainu's a hardass. He gets shit done, like a boss.
