Note that a spring is usable as a mikva for some purposes (e.g., a zav or m'tzora's dipping) and in some conditions (viz, with fewer than forty s'a and/or while not pooled) that a mikva is not. Sometimes, though, stuff can happen to spring water, after it has come out of the earth, that renders it no longer considered spring water. For example, Mishnayos Mikvaos 5:2 says:
העבירו על גבי כלים או על גבי ספסל רבי יהודה אומר הרי הוא כמו שהיה רבי יוסי אומר הרי הוא כמקוה ובלבד שלא יטביל על גבי הספסל
If he caused [spring output] to go over [the outsides of] receptacles or over a bench, R. Y'huda says it's [considered a spring] like it was and R. Yose says it's considered a mikva provided he doesn't dip on top of the bench.
Rambam rules (Yad, Mikvaos 9:10) like R. Yose.
(Rash and Rav explain that the tanaim are arguing over whether it's considered a spring so that it doesn't require forty s'a or to be pooled in order to be used as a mikva. Rosh explains that they're arguing over whether it's considered a spring so that it can be used by a zav or a m'tzora.)
The problem here is that one cannot dip in a receptacle. That, Rosh explains, is why R. Yose says the rabbis decreed that one cannot consider this water a spring. And that, too, Rosh and Rav explain, is why one cannot dip on top of the bench.
I can't pretend to know much about mikvaos, but am wondering as follows. It seems that the rabbis decreed against dipping on receptacle-like things. My local mikva for kelim provides a very holey basket for people to put, e.g., cutlery in, so that they don't have to put the pieces in one at a time and so that there is less risk of dropping them to the bottom of the mikva. That seems to me to be a receptacle-like thing that people are dipping in. Is this a problem? Or why is it different from the case in the mishna?