Well I'm not the most qualified person to answer this question AT ALL, but I think people might need a little more information.
What movement are you using? Custom? Platform Movement Object? I recently made a simple underwater movement with the PMO. I just altered the gravity/velocity/etc to my liking while the player was overlapping a water tile, then return it to normal when not overlapping the water.
Edit: Oh did you mean you want to make a liquid using particles? I can't help you there, but there are people here that surely can...