I don't know exactly what duckweed is. If you are looking for something like spiorogyra or water hyacinth there are plenty of both in various ponds and rivers. The ducks on my pond love to eat water hyacinth and I collect a truck bed full of the studd from the ditches be the side of the road every time I visit Belize City.
Other pnd weeds I just pull from the river where they are abundant.
Belize Wildlife Hotline
--- For advice and reporting wildlife incidents
BTL: 0800 822 8888 --- SMART: 822 8888
Or 605 8888