This is definitely a bug, but unfortunately I'm not sure what's causing this exactly... we will try to fix this, but I'm not sure if the fix will make it into the next hotfix...
I looked a bit further into this and noticed that as soon as I swim into one of these missing 'tiles' I find myself under water and need to swim upwards to reach its (invisible) surface. It's as if the height of the surface isn't calculated correctly which might explain why it isn't rendered either. Just thought that might help track down the issue