User blog comment:Gamboy84/Story/@comment-4931810-20120711025311/@comment-1999649-20130103004938

Also, Ryan Industries wasn't making super splicers. Other than the water breathing thing, they were actually a lot weaker and less mentally stable than "normal" splicers.

They were trying to make water breathing Big Daddies because they still have to go outside Rapture to keep it relatively non-destroyed (As seen at Dionysus Resort). Thus, the Big Daddies, air breathing organisms, had to wear extremely bulky and restricting suits, thus reducing their effectiveness when they had to fight. If they could breathe the water, then they would be able to wear armour better designed for combat without reducing their effectiveness at their other jobs.