Underwater exhausts and gas turbines really turbines really doesn't mix all that well. The only ship I'm aware of that uses it is the Swedish Visby class corvette.
In order to power your ship along, you'll need quite a bit more power, so the exhaust has to be quite a bit larger, which will take up a lot of space in the hull, right were you'll need it the most.
If you have CODAG propulsion you're also likely to see some "interesting" situations in heavy weather when external pressure becomes higher than the exhaust pressure from the diesels.
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