Okay.

First off. If Sonic is above normal ground, If the player presses the B button while sonic is jumping then sonic will do his ground pound move like normal.

If Sonic is above water, If the player presses the B button than sonic will ground pound through the water like normal.
If the player is holding down B than sonic will ground pound on the top of the water and then stand on top of the water after he has landed.
If the player isn't holding B, the player will just fall through the water like normal.

Now, if Sonic is below water, If the player presses the B button while sonic is jumping then sonic will do his ground pound move like normal.
If the player holds the up button, sonic will slowly rise to the top of the water. You are able to move back and forth, but you can't jump or roll in this state. Once you reach the top, you start to bob up and down until you hold the B button to stand on the water or let go of the up button to fall back down.