I can confirm this. The finger looked as if it were taken off by more unconventional means, such as the explosion that happened in the bathrooms. It wasn't a clean cut.

I found it on the ground during the flooding and properly put it on ice. ...Well, as properly as you can in a ziploc bag. But suffice to say, the student council president has all her fingers. This one is from the pipes. Another thing of note: while I was exploring the bathrooms with Ibara-san and Bakugo-san, we found clumps of short, black hair in the pipes as well. If anything, we might have to check around for a water tank and go from there.

Bodies in water don't decompose quickly, so that's definitely another reason why we keep seeing the pink water around.