I see no problem with this...... I used to use the rubber-maid tubs
for cheap breeding aquaria.... one side cut out with a slat of glass
or plexi-glass glued in for observation from the side. I see no
reason why they can't be used outdoors.... If your looking for
something a lot more sturdy... check your local hardware/feed store
for stock-tanks or feeding troughs.... they are much thicker, more UV
resistant, and generally better in every way.... some even come with
removable drain plugs. We use large 300 gallon stock tanks at my
workplace for keeping koi, goldfish, and floating plants during pond
season.

Good luck in your pond-keeping endeavors.