Steam is actually nicely visible, if its there. Try using some steam arrows and you will see.
Its a white-ish cloud that is usually as big as poison cloud.

This particular problem does not happen because of steam but because the water does not extinguish all of the fire. You can often see a few smaller patches still burning -under the water.
And sometimes there isnt any visible - but you do get burned when you step on that particular small space.

Which would not be the case if it was due to some steam. Because then, you would get burned in a bigger wider area.

This is visible in these areas and in the Burning legion location.

Sometimes these patches get eventually extinguished by second water barrels and rain spells - which takes quite some time, sometimes not even then.