User blog comment:WonderfulUnicorn/How the hell can he speak with a f**kin sword in his mouth/@comment-5387880-20121222122027/@comment-34954086-20121224052235

Possible. You just have to adjust the frequency of your ears (an obvious application of haki) and or voice to make up for the audioconductivity of the water which is higher then air. No problem. ;)