Do you mean new Super Mario bros wii? Because Super Mario bros wii is differeant. Oh, well, in both games don't press anything and you will slowly go down.
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for the super Mario bros wii press the A button to swim
yes
To dive you simply push down on the control stick and swim forward. Pushing the B button does not work as well because that makes Mario grab at whatever is below him.
Jump into deep water and repeatedly press the 2 Button (A Button if you are using the Nunchuk) to swim up. If you have a penguin suit, you have to hold the direction you want to swim in and repeatedly press the 2 or A button.
That hole is the exit you come out of if you die in the secret level under the black brick, so you can come out of the hole, but you can't go in.