Never underestimate household materials
From the collection of the U.S. Army Ordnance Training and Heritage Center, Ft. Lee:
This is a Vietnamese, homemade, floating water mine that was made circa 1970. It is constructed of a cardboard box wrapped in black plastic. There are nine aqua and white colored plastic flotation devices with wrapped charge simulated in between. It is held together with four wooden sticks that are tied together with rubber strips and cordage. Also, there is approximately 100 ft. of cord for directing purposes. During the Vietnam conflict, there were many of these types of devices employed in the rivers and canals of South Vietnam.
If it seems silly, keep in mind that a team of VC waterborne sappers were able to mine the WWII-era MSTS-manned jeep carrier USNS Card and put her on the bottom of Saigon harbor, although she was soon refloated and repaired.