Just for fun and flavor, I'd totally be willing to houserule that a fire elemental can walk across the surface of water, and here's why:
Fire elementals are really, really, really light. Even a huge one only weighs 8 pounds. I'm absolutely willing to believe that a fire elemental attempting to put his foot down on some water can produce enough steam to stay out of it, like a firewalker can create a barrier of steam between their feet and the hot coals simply through sweat evaporating. The effect with a fire elemental would be much greater, and the amount of upward force needed by the steam would be significantly less.
The caveat I would make to this is that the water would have to be calm. If he got hit by a wave, all bets are off.