Version: cc-tweaked-1.15.2-1.86.2
Forge: Minecraft Version: 1.15.2 (modded) forge-31.1.27

Bug: Submerge a turtle in water and have it place a block. Strangely, this seems to be ok if it's replacing a block (even if waterlogged).

Once I broke the block under the turtle, I was able to place a block down - even when waterlogged.

After more testing - it seems if the block 2 above or below is solid - the turtle will place the block.

Before:

After: