You need to place air plate here (red circle) then back to question mark in green circle and swim to '!'.
Do you have any half plate yet? There are 2 half plate to collect
> Left-half from 2nd lv. Dungeon at 2,3. Then, go back to Rat king and talk about a half plate with Tao.
> Right-half from talking with Rat king about half plate.
> Full Air plate from combine Left and Right half plate. Check on inventory>>Misc.>> click 'use' at any half plate to combine.