Something done placidly happens in a calm manner. You floated placidly, blissfully unaware of the rapids just downstream. I hope you brought your life jacket!
The word placidly is the adverb form of placid, which means "quiet and serene." Some words don't change much through their evolution. So it is with placidly, which finds its origin in the Latin placidus which means — what else? — calm and gentle. We can see the word’s meaning in the opening of the poem “Desiderata,” by Max Ehrmann: "Go placidly amid the noise and the haste, and remember what peace there may be in silence."