In spite of their 'review' of TPM, Gamespot has a pretty good walkthrough of the game at:
http://www.gamespot.com/features/tpm_gg/index.html)
I'm not sure what level you're on, but if it's the Gardens of Theed level (the one after the Otoh Gunga underwater city), then I think I know what you're going through. After filling up the 'tank' with water, swim across to the left, then up the stairs to let the water back down with another button, then walk down to where the water drained out and press the button that extends the bridge.