I don't think this counts as an Alternate Effect, honestly. If it did, you'd be swimming FASTER than your ranks of Flight instead of slower (the Extra states you're moving at your Flight speed -2). I'm thinking it's simply an extra on your flight which allows you to use it in a liquid environment with a small modifier. Therefore, I don't think you're switching arrays and aren't in need of an action to switch environments.
Flight is 2pp per rank, Swimming is only 1pp per rank. If you were using an Alternate Effect, you'd have twice as many ranks of Swimming as Flight.
James Dawsey, Owner and Publisher of Vigilance Press
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