The bug

You can not upgrade the level of the enchanting table when you place it in the ocean (surrounded by water) even with the bookshelves surrounding it. Nothing is blocking the enchanting table from upgrading, only the water (flowing water is probably included).

How to reproduce

1. Place the enchanting table in the water (it will show air, but that's in MC-125351)
2. Place the bookshelves as you would to create a max level enchanting table (15 is what you need, build it like the picture is shown)
3. Go click the enchanting table and put a non-enchanted item in the slot (like a diamond sword)
   → The enchanting table doesn't upgrade levels in the water, will not be level 30 in the last row