Here's why:
* Gerund: A gerund is a verb that acts as a noun. It ends in "-ing".
* Gerund Phrase: A gerund phrase is a group of words that includes a gerund and its modifiers (words that describe or add information to the gerund).
In this sentence, "Floating down river on a raft" acts as the subject of the sentence, performing the action of being "not an idea of fun".