Boethiah's voice is distorted underwater
Boethiah's voice should not be affected underwater. Technically, it is the player who is speaking:
Player->Say "Vo\Misc\DA_BoethiahIntro1.wav", "What do you want with me, mortal? My shrine is in ruins, my priests have forgotten me. Will you rebuild my shrine, mortal? Restore it in all its glory? Accomplish this, and I will reward you. You will be the bearer of Goldbrand, my sword of legend. There is one who can help you do this for me. Listen...."
Vanilla (unlike OpenMW) doesn't seem to apply environmental effects to player's voice.