User blog comment:FairyRaven/Fairy Tail Chapter 397: Steel, Review and Commentary/@comment-25111584-20140826200458/@comment-7418318-20140826203731

Your forgot about the pressure of the water. Even if you are just right below the surface, the water still has a lot more pressure than air, so you'd need to expell a lot more air than you usually would while taking to prevent said water from flowing back down your throat and straight into your lungs.

And while I haven't tried it, I could imagine it's quite hard to get any sounds out, much less sounds accurate enough to be understood by someone else.

On top of that you'd also run out of breath very quickly like that.