While Nitori's case is rather typical, the Marisa's case is hilariously over the top to the point of making me wonder "how the fuck she did that with only 1 sharp thing!?"
At first I thought it's supposed to be a hook so the Marisa can dock their hat. Like in that raising aquatic Marisa story
These sharp objects could harm the pet yukkuris as a result of negligence, no matter if it's due to the owner or the yukkuris themselves.
In addition to this, yukkuris possess zero tolerance to pain; even the slightest stimulus could cause over-reaction. It is thus more fatal if the yukkuris are hurt in water.