This was the explanation from @lucadeltodecso
lucadeltodecsoDeveloper
It being transparent would also cause issues with fogging. Underwater will have seperate fog parameters to outside water soon, and when the camera is in water it will change to ‘water fog’, but if you could see straight into the water from outside without the ‘waterfall’ effect it’d often look totally broken fog-wise
Personally I could easily live with there being no underwater fog when viewed through glass as the current situation looks even more broke to me.