Absent a specific reference to fire elementals taking damage from water, I'd rule that the elemental who came up short on his jump check would be shunted back to the edge he jumped from (even if he had jumped 38' across a 40' river). There's be a lot of steam and some hissing curse words in whatever language fire elementals speak, but I'd probably draw the line there.
Now, in a game like AE which has a lot of elemental attack spells (and generally, the caster gets to pick the element), I go with opposing elements take double damage from those attacks, so a fire elemental hit with a Sorcerous Blast [Water] is pretty seriously screwed. But elemental magic is more ingrained in that system than D&D, where that doesn't come up all that much.
Or you could import an old Shadowrun rule, so that if a 8 HD fire elemental grapples a 4 HD water elemental, what you are left with is a 4 HD fire elemental. Although that probably wouldn't port to well to D&D as it would seriously nerf the higher level elementals, it would be vastly simpler.