I do not recommend top cropping into water; however, then again, I do not recommend rinsing yeast with and storing it under water because the practice does not do what home brewers have been led to believe. The reason why we especially do not want to store top-cropped yeast under water is that top-cropped yeast has not undergone the morphological changes that occur at the end of fermentation.
At the end of fermentation (i.e., when carbon becomes limiting), yeast cells store trehalose and glycogen. Their cell walls also thicken before entering a state known as quiescence. These cellular changes are a survival mechanism that allows yeast cells to withstand periods of starvation. Cellular activity does not stop during quiescence. It merely slows down like that of a hibernating animal.
The standard practice of a taking a top-crop when the gravity reaches 50% of original gravity results in the healthiest yeast crop for re-pitching, but the crop is very fragile from a storage point of view. What I do when I top crop is to feed the cropped yeast green beer from the fermentation at a rate of roughly two to three parts green beer to one part yeast. That way, we store the yeast in a low-pH solution that gives the cells an opportunity to enter quiescence. While carbon will become limiting fairly quickly, it is not like the shock experienced when the carbon source is removed abruptly.