I can answer the first question, but I’ll have to wait a day or two to get back to you on the second one. My friend spent an all-nighter making the caustics, and we’ve been spending many hours a day since then awake (we had a tech demo today for our sponsor, it went well!). Anyway I need to ask him to get insight into how he worked the light shafts.
As for the material, yes we use a material plane, when the player is underwater we simply set your water to invisible. When you reach a certain part of the Z axes you actually become a different blueprint actor and that actor’s possession sets your to visible, and sets our plane to invisible. Ideally I think you would want to modify the material, we just haven’t had enough time to learn ue4 materials yet with our project ending in 2 weeks.
I’ll be sure to ask my friend about the light shafts. In fact I might convince him to create his own account and post 
!
Cheers,