If your planned location would not be damaged by a little overflow from the barrel, this might be just the thing to add a bit more interest to your watergarden. Overwintering If you live in a cold climate like I do you will have to decide how to over-winter your barrel. I wait for the fall die-back and clip the dead plant materal away. The plants are placed at the bottom of the barrel and the barrel is refilled with water. It stays warm enough in the garage that the barrel won't freeze solid and my plants have always come through ok.
Animals have to be brought inside to a fish tank or replaced in the spring.
This is a smaller version of the floating pond heaters that are used to keep pond surfaces open in winter. Such a heater, perhaps assisted by some sort of insulation around the barrel, should keep it thawed thru the cold months. Local birds will reward you with their visits to what might be the only unfrozen water in a large area. Make sure that the heating element doesn't contact the plastic liner or it may melt. Check with your local city government for code requirements.