Technically water already has the exact same problems glass would have if it were semi-transparent. When you have multiple overlapping layers of water visible, you can get some really funky looking rendering. We made it less funky by having water write to depth even though its semi-transparent… but that introduced other problems that it screws up various other things like not being able to see particle effects through water or making the volumetric fog interact strangely when water is in the background. So we “could” make glass semi-transparent with those same gotchas and glitches, though it would still require an extra mesh per chunk compared to now to be rendered. The main reason you dont generally notice this stuff with the water, is the water “tends” to be a single flat plane, without multiple overlapping plains of water, and without much going on “under” the water for you to notice the problems… but glass would not be used like that, so you “would” notice the problems straight away.
One thing I would really like to experiment with at some point given time (time is never available…) is with an overhaul of the rendering pipeline to experiment with order-independent transparency solutions which would let us have “proper” semi-transparent water as well as semi-transparent glass which wouldn’t have any glitchy problems though would still be more expensive compared to the water now.
(Equally I’d like at some point to experiment with screen-space reflections so that we can have water reflect at any height in the world, and to allow reflections on specular objects and even blocks. Of course screen-space reflections aren’t perfect and things that are reflected right now in the water, would not necessarigly be reflected any more. Eg right now you can see planets reflected in the water even if the you cannot see the planet on the screen otherwise which would no longer work. Trade offs.)
(Equally I’d like at some point to experiment with further overhauls of the rendering engine to get better performance with more deferred rendering, but now we’re really talking time sinks…)