Maupassant is known primarily for his numerous short stories, and also for his half dozen novels, but he also published non-fiction, including this short travel memoir, in 1888. It's a curiosity, being a mixture of anecdote, polemic, meditation and a small amount of travel writing.
After the success of Bel-Ami, and his increasing popularity as a short-story writer, Maupassant had money to indulge himself, and bought a yacht, which he named after the novel. This book was supposedly written on a Mediterranean trip on the yacht, over a period of 8 days in 1887, and is in a diary or memorandum format. Douglas Parmee, however, the venerable translator points out in his introduction that he'd used some of the anecdotes before, and it was in his nature to revise his works, so the illusion of spontaneity here is one of Maupassant's techniques.
It's an odd work, with no great coherence - there are rants against warmongers, amusing anecdotes, personal fears and reflections, it's both light and dark, shallow and deep. The title is obviously punning (even in French 'Sur l'eau') suggesting the drifting style of the narrative, putting into many ports as the captain's whim decides.
There are, as you might expect from Maupassant, passages of great lyrical beauty, and also of poignant observation, and it exposes aspects of his character in a direct way that the stories only hint at. He is, for example, nervous of crowds - he spend several pages explaining why he avoids them. This may well be a symptom of his incipient psychosis, caused by syphilis. Within three years of writing Afloat, he was considered insane, and he died a couple of years after that. As his mind deteriorated, he became obsessed in his fiction with supernatural elements, which were representation of the demons he was assailed by. The greatest poignancy of this book is that it is a late personal glimpse of Maupassant, cresting high on his fame, but with a dark storm on the horizon.