The boat is made of 10 tonnes of shredded plastic waste, moulded and compacted to form the hull, keel and ribs with only the mast made out of wood.
UNEP estimates that more than 8.3 billion tonnes of plastic has been produced globally since the early 1950s.
Kenya is battling the scourge of plastic pollution, which chokes turtles, cattle, and birds and blights the landscape.
The boat is expected to sail from Lamu to Zanzibar on February 7.