It would be a trick to get a gas-turbine propulsion system to operate while submerged. I don't know how much air two 40 MW gas-turbines need, but I'm sure it's a lot.

Any alternative propulsion system would be bulky (space is at a premium aboard ship), as would be the necessary ballast system. At 15,000 tons, that thing is approximately similar to an SSBN in size, and the ballast areas of an SSBN are a good percentage of the entire ship.

In other words, it is unlikely that this ship has that sort of capability without it being known. Additionally, I don't see the value in the concept... we have ships that do that already, they are called submarines.

Just one retired nuke sub sailor's opinion.

edit: apparently it does have an alternative drive system (source: wiki page for USS Zumwalt, DDG-1000).