A situation occurred where a cheeky Bard spoke afoul one too many times in the presence of some seriously bad hombres, and as a result, he got half his tongue cut out.
Now I'm not familiar with how much having a partial tongue interferes with speaking, but the table argued that he can say some words with great difficulty.
This thread in particular that goes into the details of casting while underwater concludes that you ultimately can, RAW, cast spells underwater, arguing that the sound produced in your larynx is the same sound produced above water, it just becomes garbled as soon as it hits the water.
Moreover, here's what the PHB says about verbal components:
Most spells require the chanting of mystic words. The words themselves aren’t the source of the spell’s power; rather, the particular combination of sounds, with specific pitch and resonance, sets the threads of magic in motion. Thus, a character who is gagged or in an area of silence, such as one created by the silence spell, can’t cast a spell with a verbal component.
So, we need Mystic Words to be spoken. Does pronunciation affect the casting? How much does intelligibility affect the casting? To extrapolate this, do characters with poor accents suffer in a similar regard if our bard cannot cast verbally due to his terrible pronunciation? The thread above dictated that you don't even need to be understood, simply the correct pitch and resonance needs to be met.
Is the bard doomed or can he mumble his way around the battlefield?