see if there are already existing keys (perhaps undocumented). In this case, they are most likely to be under attraction=* so its taginfo might help (e.g. there seem to be few instances of attraction=inflatable which might be applicable; I’ve never seen those buckets so have no idea what they might be called like)
if they are tagged under some other key and not attraction=* (not likely, but possible), I’d try looking what tags are used inside existing leisure=water_park polygons (ideally find some areas suspect are nicely mapped and have that feature), e.g. using overpass query like this
and then if all that fails and nobody gives better ideas here,
I’d try to find out how they are commonly named by searching the web, and then (in this particular case) I’d just use ATYL (in other cases like new keys instead of values, or not straightforward, I’d likely strongly prefer more demanding Proposal process to them up. Please do document them at wiki if you do add them. Suggesting: use a value with water_ in it.
Alternatively (and much more easily) you can simply add human-readable description=* on the leisure=water_park polygon itself motioning facilities there, without micromapping them one by one.