I agree with PD. I would rule that someone could be pulled under the surface and if they we're let go, I'd let then immediately rise back. I believe we house ruled it in our own games that water walk allowed you kneel and such on the surface and the spell generally provided a barrier, so if prone, the character would still be just over the surface.
As for being pulled down in a grapple situation, I would grant the character a +4 (+8 if you're generous, it is a force raising you up 10 ft/sec) to resist being pulled directly down (the move grapple action), as there is a "buoyant" force pulling him directly up.
Unless the liquid was an acid or heat hazard, I wouldn't rule it as a hazardous location.