casual water – “a temporary accumulation of water on the golf course”. Technically the accumulation of water has to be above ground and visible, and this does not include lakes; wet, spongy, mushy or muddy ground; dew or frost, or snow or ice [source].

So in other words casual water is another way of describing a puddle, a diminutive of the Old English word pudd (ditch) which originally referred to pools and ponds as well [source].