I got these for my daughter when she was about 18mo (I previously got the 18mo size and re-purchased as my daughter outgrew them).  She uses it for 'water day' at daycare where they run on foamy play-turf with sprinklers on.  Any rougher surface like a water park will probably eat up the bottoms of these are they do not have a sole.  Now that my daughter is 2 years old and running around everywhere, I find baby aquasox/water shoes like those made by i-play which have real soles are much better for outside use.  These are easy to get on and off even when wet so are good for the pool.  They didn't seem to be slippery even when wet, even on wet tile surfaces.