Kingdom Hearts Mermaid Kick

I'm seriously stuck on KH. I cannot use Mermaid Kick properly to get through the current in Atlantica to take on Big Ursula. Can somehow tell me how to use Mermaid Kick properly and how to get down the current channel without being shoved back out in seconds? And is Mermaid Kick supposed to go upwards?

 

Lets see if i remember. I think you press either X or Square twice really quick (or whatever button it is to swim up or down) & then just hold it in.

 

I thought you just kept hitting X or Square

 

No, no, I most recently finished Kingdom Hearts for the second time, and Mermaid Kick was actvated by pressing the 'O' button twice. That is if you have it equipped. You may or may not have it equipped, which is causing the trouble, or you may not know the button. Either way, I hope this helps you.