This is a new native wetland plant for me. I'm so excited! It's the creeping buttercup, probably Ranunculus reptans, found growing on a stump on a kayaking expedition.
It's a tiny plant, easily overlooked - maybe just as well, since all parts are considered toxic if ingested. Apparently touching it also causes unstable ranunculin in the plant to turn into an oily toxin, so the sap may cause contact dermatitis.
According to Wikipedia it has some adaptation to rain pollination, of which I know nothing, except to guess that rain helps spread the pollen? There isn't much scientific evidence of this being the case.