I was able to replicate your bug by creating a new landscape and just adding a lake: when I move the camera to the lake floor or just under the waves, I didn’t see any underwater post processing effects in those areas. There is a “Enabled” checkbox for the lake’s “Underwater Post Process Settings” , but it’s already checked… and unchecking it just removed the effect entirely. The only fix I found was adding an ocean to the landscape and set the splines so that it’s outside the landscape (so it doesn’t deform any part of the terrain) - this automatically fixed the bug without having to change any collision extents.
However, this fix is limited to lake depths above [about] 30,000 (should be fine for most lakes!), as it doesn’t seem to render the underwater effect below that. Even if I changed the Z Collision Extent for the ocean to 100,000, nothing changed… in fact, no matter what I changed the Z Collision Extent to, it didn’t seem to make any difference. Only changing the X and Y made a difference (or at least a difference I could see, as the post processing box for the ocean changed when I changed the X and Y, but not when I changed Z). From what I see, changing the Z collision extents is probably a bug in the plugin itself.
Tl;dr: add an ocean around your entire landscape (unless you already have one), and that should fix the error as long as your lake isn’t just a really thin, deep ocean.