The standard approach to underwater audio in games is a low-pass filter over the normal air-propagation model — turn down the highs, call it done. It reads as underwater to most players for about thirty seconds.
Real underwater sound propagation is a genuinely different physical system: sound travels roughly four times faster, attenuates differently by frequency, and reflects off a water surface in ways that don’t map onto any air-based reverb model. For Undertow we ended up building a separate propagation path rather than bolting a filter onto the existing one.
It’s more work, and most players will never consciously notice the difference. But the ones who’ve actually been underwater will feel that something is finally right.