When you tap the crouch button under water you still able to fall on the edge and use functional blocks and also the paper doll doesn't pop up 

Expected result:

After clicking sneak/crouch button:

• The paper doll will show that you are crouching
• You can't use functional blocks while holding items in your main hand.
• You don't get damage from magma blocks

Actual result:

After clicking sneak/crouch button:

• The paper doll doesn't show that you are crouching
• You can use functional blocks while holding items in your main hand
• You get damage from magma blocks